mac: Make build documentation less out-of-date.

docs/mac_build_instructions.md

 # Mac Build Instructions
 
 [TOC]
 
 ## Prerequisites
 
 *   A Mac running 10.9+.
-*   [Xcode](https://developer.apple.com/xcode) 5+.
+*   [Xcode](https://developer.apple.com/xcode) 7.3+.
 *   [depot\_tools](http://dev.chromium.org/developers/how-tos/depottools).
-*   Xcode Command Line Tools. Run
-    ```
-    xcode-select --install
-    ```
-    to install them.
 *   The OSX 10.10 SDK. Run
     ```
     ls `xcode-select -p`/Platforms/MacOSX.platform/Developer/SDKs
     ```
-    to check whether you have it. If you don't have it, you need to get one from
-    an install of Xcode 6, and place it in the above directory.
+    to check whether you have it.  Building with the 10.11 SDK works too, but
+    the releases currently use the 10.10 SDK.
 
 ## Getting the code
 
 [Check out the source code](https://www.chromium.org/developers/how-tos/get-the-code)
 using Git.
 
 Before checking out, go to the
 [waterfall](http://build.chromium.org/buildbot/waterfall/) and check that the
 source tree is open (to avoid pulling a broken tree).
 
 The path to the build directory should not contain spaces (e.g. not
 `~/Mac OS X/chromium`), as this will cause the build to fail. This includes your
 drive name, the default "Macintosh HD2" for a second drive has a space.
 
 ## Building
 
-Chromium on OS X can only be built using the [Ninja](ninja_build.md) tool and
+Chromium on OS X is built using the [Ninja](ninja_build.md) tool and
 the [Clang](clang.md) compiler. See both of those pages for further details on
 how to tune the build.
 
+Run
+
+    gn gen out/gn
+
+to generate build files (replace "gn" in "out/gn" with whatever you like), and
+then run
+
+    ninja -C out/gn chrome
+
+to build.  You can edit out/gn/args.gn to configure the build.
+
 Before you build, you may want to
 [install API keys](https://sites.google.com/a/chromium.org/dev/developers/how-tos/api-keys)
 so that Chrome-integrated Google services work. This step is optional if you
 aren't testing those features.
 
-### Raising system-wide and per-user process limits
-
-If you see errors like the following:
-
-```
-clang: error: unable to execute command: posix_spawn failed: Resource temporarily unavailable
-clang: error: clang frontend command failed due to signal (use -v to see invocation)
-```
-
-you may be running into too-low limits on the number of concurrent processes
-allowed on the machine. Check:
-
-    sysctl kern.maxproc
-    sysctl kern.maxprocperuid
-
-You can increase them with e.g.:
-
-    sudo sysctl -w kern.maxproc=2500
-    sudo sysctl -w kern.maxprocperuid=2500
-
-But normally this shouldn't be necessary if you're building on 10.7 or higher.
-If you see this, check if some rogue program spawned hundreds of processes and
-kill them first.
-
 ## Faster builds
 
 Full rebuilds are about the same speed in Debug and Release, but linking is a
 lot faster in Release builds.
 
-Run
+Put
 
-    GYP_DEFINES=fastbuild=1 build/gyp_chromium
+    is_debug = false
 
-to disable debug symbols altogether, this makes both full rebuilds and linking
-faster (at the cost of not getting symbolized backtraces in gdb).
+in your args.gn to do a release build.
+
+Put
+
+    is_component_build = true
+
+in your args.gn to build many small dylibs instead of a single large executable.
+This makes incremental builds much faster, at the cost of producing a binary
+that opens less quickly.  Component builds work in both debug and release.
+
+Put
+
+    symbol_level = 1
+
+in your args.gn to disable debug symbols altogether.  This makes both full
+rebuilds and linking faster (at the cost of not getting symbolized backtraces
+in gdb).
 
 You might also want to [install ccache](ccache_mac.md) to speed up the build.
 
 ## Running
 
 All build output is located in the `out` directory (in the example above,
 `~/chromium/src/out`).  You can find the applications at
-`{Debug|Release}/ContentShell.app` and `{Debug|Release}/Chromium.app`, depending
-on the selected configuration.
+`gn/Content Shell.app` and `gn/Chromium.app`.
 
 ## Unit Tests
 
 We have several unit test targets that build, and tests that run and pass. A
 small subset of these is:
 
 *   `unit_tests` from `chrome/chrome.gyp`
 *   `base_unittests` from `base/base.gyp`
 *   `net_unittests` from `net/net.gyp`
 *   `url_unittests` from `url/url.gyp`
 
-When these tests are built, you will find them in the `out/{Debug|Release}`
+When these tests are built, you will find them in the `out/gn`
 directory. You can run them from the command line:
 
-    ~/chromium/src/out/Release/unit_tests
+    ~/chromium/src/out/gn/unit_tests
 
 
 ## Coding
 
 According to the
 [Chromium style guide](http://dev.chromium.org/developers/coding-style) code is
 [not allowed to have whitespace on the ends of lines](https://google.github.io/styleguide/cppguide.html#Horizontal_Whitespace).
-If you edit in Xcode, know that it loves adding whitespace to the ends of lines
-which can make editing in Xcode more painful than it should be. The
-[GTM Xcode Plugin](http://code.google.com/p/google-toolbox-for-mac/downloads/list)
-adds a preference panel to Xcode that allows you to strip whitespace off of the
-ends of lines on save. Documentation on how to install it is
-[here](http://code.google.com/p/google-toolbox-for-mac/wiki/GTMXcodePlugin).
+
+Run `git cl format` after committing to your local branch and before uploading
+to clang-format your code.
 
 ## Debugging
 
 Good debugging tips can be found
 [here](http://dev.chromium.org/developers/how-tos/debugging-on-os-x). If you
 would like to debug in a graphical environment, rather than using `lldb` at the
 command line, that is possible without building in Xcode. See
-[Debugging in Xcode](http://www.chromium.org/developers/debugging-on-os-x/building-with-ninja-debugging-with-xcode)
+[Debugging in Xcode](http://www.chromium.org/developers/how-tos/debugging-on-os-x/building-with-ninja-debugging-with-xcode)
 for information on how.
 
 ## Contributing
 
 Once you’re comfortable with building Chromium, check out
 [Contributing Code](http://dev.chromium.org/developers/contributing-code) for
 information about writing code for Chromium and contributing it.
 
 ## Using Xcode-Ninja Hybrid
 
-While using Xcode is unsupported, GYP supports a hybrid approach of using ninja
-for building, but Xcode for editing and driving compilation. Xcode can still be
-slow, but it runs fairly well even **with indexing enabled**.
+While using Xcode is unsupported, gn supports a hybrid approach of using ninja
+for building, but Xcode for editing and driving compilation.  Xcode is still
+slow, but it runs fairly well even **with indexing enabled**.  Most people
+build in the Terminal and write code with a text editor though.
 
 With hybrid builds, compilation is still handled by ninja, and can be run by the
-command line (e.g. ninja -C out/Debug chrome) or by choosing the chrome target
+command line (e.g. ninja -C out/gn chrome) or by choosing the chrome target
 in the hybrid workspace and choosing build.
 
-To use Xcode-Ninja Hybrid, set GYP\_GENERATORS like the following:
+To use Xcode-Ninja Hybrid pass --ide=xcode to `gn gen`
 
 ```shell
-export GYP_GENERATORS="ninja,xcode-ninja"
+gn gen out/gn --ide=xcode
 ```
 
-Due to the way Xcode parses ninja output paths, it's also necessary to change
-the main gyp location to anything two directories deep. Otherwise Xcode build
-output will not be clickable.
-To make this change permanent, you can edit `chromium.gyp_env` (or create it if
-it does not exists) and define GYP\_GENERATOR\_FLAGS. In general, to use hybrid
-mode, your `chromium.gyp_env` could contain the following:
-
-```json
-{
-  "GYP_GENERATORS"     : "ninja,xcode-ninja",
-  "GYP_GENERATOR_FLAGS":
-      "xcode_project_version=3.2 " +
-      "xcode_ninja_main_gyp=src/build/ninja/all.ninja.gyp",
-}
-```
-
-After, generate the project files with:
+Open it:
 
 ```shell
-gclient runhooks
-```
-
-And finally, open it:
-
-```shell
-open build/ninja/all.ninja.xcworkspace
+open out/gn/ninja/all.xcworkspace
 ```
 
 You may run into a problem where http://YES is opened as a new tab every time
 you launch Chrome. To fix this, open the scheme editor for the Run scheme,
 choose the Options tab, and uncheck "Allow debugging when using document
 Versions Browser". When this option is checked, Xcode adds
 `--NSDocumentRevisionsDebugMode YES` to the launch arguments, and the `YES` gets
 interpreted as a URL to open.
 
-If you want to limit the number of targets visible, which is known to improve
-Xcode performance, add `xcode_ninja_executable_target_pattern=%target%` where
-`%target%` is a regular expression matching executable targets you'd like to
-include.
-
-To include non-executable targets, use `xcode_ninja_target_pattern=All_iOS`.
-
 If you have problems building, join us in `#chromium` on `irc.freenode.net` and
 ask there. As mentioned above, be sure that the
 [waterfall](http://build.chromium.org/buildbot/waterfall/) is green and the tree
 is open before checking out. This will increase your chances of success.
 
-## Using Emacs as `EDITOR` for `git commit`
-
-Using the [Cocoa version of Emacs](http://emacsformacosx.com/) as the `EDITOR`
-environment variable on Mac OS will cause `git commit` to open the message in a
-window underneath all the others. To fix this, create a shell script somewhere
-(call it `$HOME/bin/EmacsEditor` in this example) containing the following:
-
-```
-#!/bin/sh
-
-# All of these hacks are needed to get "git commit" to launch a new
-# instance of Emacs on top of everything else, properly pointing to
-# the COMMIT_EDITMSG.
-
-realpath() {
-    [[ $1 = /* ]] && echo "$1" || echo "$PWD/${1#./}"
-}
-
-i=0
-full_paths=()
-for arg in "$@"
-do
-  full_paths[$i]=$(realpath $arg)
-  ((++i))
-done
-
-open -nWa /Applications/Emacs.app/Contents/MacOS/Emacs --args --no-desktop \
-    "${full_paths[@]}"
-```
-
-and in your `.bashrc` or similar,
-
-    export EDITOR=$HOME/bin/EmacsEditor
-
 ## Improving performance of `git status`
 
 `git status` is used frequently to determine the status of your checkout.  Due
 to the number of files in Chromium's checkout, `git status` performance can be
 quite variable.  Increasing the system's vnode cache appears to help.  By
 default, this command:
 
     sysctl -a | egrep kern\..*vnodes
 
 Outputs `kern.maxvnodes: 263168` (263168 is 257 * 1024).  To increase this
@@ skipping 23 matching lines @@
 ```
 
 the Xcode license hasn't been accepted yet which (contrary to the message) any
 user can do by running:
 
     xcodebuild -license
 
 Only accepting for all users of the machine requires root:
 
     sudo xcodebuild -license