Update Mac build instructions with GN

docs/mac_build_instructions.md

 # Mac Build Instructions
 
 [TOC]
 
 ## Prerequisites
 
 *   A Mac running 10.9+.
 *   [Xcode](https://developer.apple.com/xcode) 5+.
 *   [depot\_tools](http://dev.chromium.org/developers/how-tos/depottools).
 *   Xcode Command Line Tools. Run
@@ skipping 54 matching lines @@
 
 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
+Add
 
-    GYP_DEFINES=fastbuild=1 build/gyp_chromium
+    symbol_level=1
 
-to disable debug symbols altogether, this makes both full rebuilds and linking
-faster (at the cost of not getting symbolized backtraces in gdb).
+to your [GN](../tools/gn/docs/quick_start.md) config 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.
 
@@ skipping 33 matching lines @@
 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)
 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
+## Using Xcode
 
-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**.
-
-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
-in the hybrid workspace and choosing build.
-
-To use Xcode-Ninja Hybrid, set GYP\_GENERATORS like the following:
-
-```shell
-export GYP_GENERATORS="ninja,xcode-ninja"
-```
-
-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:
-
-```shell
-gclient runhooks
-```
-
-And finally, open it:
+Use the `--ide="xcode"` when running `gn gen` to generate the workspace, then
+open it:
 
 ```shell
 open build/ninja/all.ninja.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

[vabr (Chromium), 2016/08/09 13:13:40]

I suppose that this might still be needed, but not sure how it translates to GN.
Any ideas?

-`%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`.

[vabr (Chromium), 2016/08/09 13:13:40]

Ditto.

-
 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
@@ skipping 62 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