More fixes to the build instructions.

docs/mac_build_instructions.md

-# Mac Build Instructions
+# Checking out and building Chromium for Mac
 
-Google employee? See [go/building-chrome](https://goto.google.com/building-chrome) instead.
+There are instructions for other platforms linked from the 
+[get the code](get_the_code.md) page.
+
+**See also [the old version of this page](old_mac_build_instructions.md).**
+
+## Instructions for Google Employees
+
+Are you a Google employee? See
+[go/building-chrome](https://goto.google.com/building-chrome) instead.
 
 [TOC]
 
 ## System requirements
 
 *   A 64-bit Mac running 10.9+.
 *   [Xcode](https://developer.apple.com/xcode) 7.3+.
 *   The OS X 10.10 SDK. Run
 
     ```shell  
@@ skipping 48 matching lines @@
 
 ```shell
 $ cd src
 ```
 
 *Optional*: You can also [install API
 keys](https://www.chromium.org/developers/how-tos/api-keys) if you want your
 build to talk to some Google services, but this is not necessary for most
 development and testing purposes.
 
-## Building
+## Setting up the build
 
 Chromium uses [Ninja](https://ninja-build.org) as its main build tool along
 with a tool called [GN](../tools/gn/docs/quick_start.md) to generate `.ninja`
 files. You can create any number of *build directories* with different
 configurations. To create a build directory:
 
 ```shell
 $ gn gen out/Default
 ```
 
@@ skipping 38 matching lines @@
 ```
 symbol_level = 0
 ```
 
 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.
 
+## Build Chromium
+
+Build Chromium (the "chrome" target) with Ninja using the command:
+
+```shell
+$ ninja -C out/Default chrome
+```
+
+You can get a list of all of the other build targets from GN by running `gn ls
+out/Default` from the command line. To compile one, pass the GN label to Ninja
+with no preceding "//" (so, for `//chrome/test:unit_tests` use `ninja -C
+out/Default chrome/test:unit_tests`).
+
 ## Run Chromium
 
 Once it is built, you can simply run the browser:
 
 ```shell
 $ out/Default/chrome
 ```
 
 ## Running test targets
 
 You can run the tests in the same way. You can also limit which tests are
 run using the `--gtest_filter` arg, e.g.:
 
 ```
-$ ninja -C out/Default unit_tests --gtest_filter="PushClientTest.*"
+$ out/Default/unit_tests --gtest_filter="PushClientTest.*"
 ```
 
 You can find out more about GoogleTest at its
 [GitHub page](https://github.com/google/googletest).
 
 ## 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
@@ skipping 101 matching lines @@
 
 ```shell
 $ xcodebuild -license
 ```
 
 Only accepting for all users of the machine requires root:
 
 ```shell
 $ sudo xcodebuild -license
 ```