Add the missing Cast build instructions.

docs/linux_cast_build_instructions.md

+# Checking out and building Cast on Linux
+
+**Note**: it is **not possible** to build a binary functionally
+equivalent to a Chromecast. This is to build a single-page content
+embedder with similar functionality to Cast products.
+
+## Instructions for Google Employees
+
+Are you a Google employee? See
+[go/building-linux-cast](https://goto.google.com/building-linux-cast) instead.
+
+[TOC]
+
+## System requirements
+
+*   A 64-bit Intel machine with at least 8GB of RAM. More than 16GB is highly
+    recommended.
+*   At least 100GB of free disk space.
+*   You must have Git and Python installed already.
+
+Most development is done on Ubuntu (currently 14.04, Trusty Tahr). There are
+some instructions for other distros below, but they are mostly unsupported.
+
+## Install `depot_tools`
+
+Clone the `depot_tools` repository:
+
+```shell
+$ git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
+```
+
+Add `depot_tools` to the end of your PATH (you will probably want to put this
+in your `~/.bashrc` or `~/.zshrc`). Assuming you cloned `depot_tools` to
+`/path/to/depot_tools`:
+
+```shell
+$ export PATH="$PATH:/path/to/depot_tools"
+```
+
+## Get the code
+
+Create a `chromium` directory for the checkout and change to it (you can call
+this whatever you like and put it wherever you like, as long as the full path
+has no spaces):
+
+```shell
+$ mkdir ~/chromium && cd ~/chromium
+```
+
+Run the `fetch` tool from depot_tools to check out the code and its
+dependencies.
+
+```shell
+$ fetch --nohooks chromium
+```
+
+If you don't want the full repo history, you can save a lot of time by
+adding the `--no-history` flag to `fetch`.
+
+Expect the command to take 30 minutes on even a fast connection, and many
+hours on slower ones.
+
+If you've already installed the build dependencies on the machine (from another
+checkout, for example), you can omit the `--nohooks` flag and `fetch`
+will automatically execute `gclient runhooks` at the end.
+
+When `fetch` completes, it will have created a hidden `.gclient` file and a
+directory called `src` in the working directory. The remaining instructions
+assume you have switched to the `src` directory:
+
+```shell
+$ cd src
+```
+
+### Install additional build dependencies
+
+Once you have checked out the code, and assuming you're using Ubuntu, run
+[build/install-build-deps.sh](/build/install-build-deps.sh)
+
+You may need to adjust the build dependencies for other distros. There are
+some [notes](#notes) at the end of this document, but we make no guarantees
+for their accuracy.
+
+### Run the hooks
+
+Once you've run `install-build-deps` at least once, you can now run the
+Chromium-specific hooks, which will download additional binaries and other
+things you might need:
+
+```shell
+$ gclient runhooks
+```
+
+*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.
+
+## 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, run:
+
+```shell
+$ gn gen out/Default --args='is_chromecast=true'
+```
+
+* You only have to run this once for each new build directory, Ninja will
+  update the build files as needed.
+* You can replace `Default` with another name, but
+  it should be a subdirectory of `out`.
+* For other build arguments, including release settings, see [GN build
+  configuration](https://www.chromium.org/developers/gn-build-configuration).
+  The default will be a debug component build matching the current host
+  operating system and CPU.
+* For more info on GN, run `gn help` on the command line or read the
+  [quick start guide](../tools/gn/docs/quick_start.md).
+
+### <a name="faster-builds"></a>Faster builds
+
+You might try some of the suggestions on the
+[Linux build setup](linux_build_instructions.md#faster-builds).
+
+## Build cast\_shell
+
+Build cast\_shell with Ninja using the command:
+
+```shell
+$ ninja -C out/Default cast_shell
+```
+
+## Run cast\_shell
+
+Once it is built, you can simply run it:
+
+```shell
+$ out/Default/cast_shell --ozone-platform=x11 http://google.com
+```
+
+## Update your checkout
+
+To update an existing checkout, you can run
+
+```shell
+$ git rebase-update
+$ gclient sync
+```
+
+The first command updates the primary Chromium source repository and rebases
+any of your local branches on top of tip-of-tree (aka the Git branch
+`origin/master`). If you don't want to use this script, you can also just use
+`git pull` or other common Git commands to update the repo.
+
+The second command syncs dependencies to the appropriate versions and re-runs
+hooks as needed.
+
+## Tips, tricks, and troubleshooting
+
+### More links
+
+*   Want to use Eclipse as your IDE? See
+    [LinuxEclipseDev](linux_eclipse_dev.md).