Add component build documentation.

docs/ninja_build.md

 # Ninja Build
 
 Ninja is a build system written with the specific goal of improving the
 edit-compile cycle time. It is used by default everywhere except when building
 for iOS.
 
 Ninja behaves very similar to Make -- the major feature is that it starts
 building files nearly instantly. (It has a number of minor user interface
 improvements to make as well.)
 
 Read more about Ninja at [the Ninja home page](https://ninja-build.org/).
 
 ## Using it
 
 ### Configure your system to use Ninja
 
 #### Install
 
-Ninja is included in `depot_tools` as well as `gyp`, so there's nothing to
-install.
+Ninja is included in `depot_tools` so there's nothing to install.
 
 ## Build instructions
 
 To build Chrome:
 
     cd /path/to/chrome/src
     ninja -C out/Debug chrome
 
 Specify `out/Release` for a release build. I recommend setting up an alias so
 that you don't need to type out that build directory path.
@@ skipping 63 matching lines @@
 You can write a specific build config to a specific output directory via the
 `-G` flags to gyp. Here's an example from jamesr:
 `build/gyp_chromium -Gconfig=Release -Goutput_dir=out_profiling -Dprofiling=1
 -Dlinux_fpic=0`
 
 ## Bugs
 
 If you encounter any problems, please file a bug at http://crbug.com/new with
 label `ninja` and cc `thakis@` or `scottmg@`.  Assume that it is a bug in Ninja
 before you bother anyone about e.g. link problems.