docs: Change "ChromeOS" to "Chrome OS".

tools/gn/docs/cross_compiles.md

 # How GN handles cross-compiling
 
 ## As a GN user
 
 GN has robust support for doing cross compiles and building things for
 multiple architectures in a single build (e.g., to build some things to
 run locally and some things to run on an embedded device). In fact,
 there is no limit on the number of different architectures you can build
 at once; the Chromium build uses at least four in some configurations.
 
@@ skipping 31 matching lines @@
 
 To do an 32-bit ARM Android cross-compile, do:
 
 ```
 gn gen out/Default --args='target_os="android"'
 ```
 
 (We don't have to specify target\_cpu because of the conditionals
 mentioned above).
 
-And, to do a 64-bit MIPS ChromeOS cross-compile:
+And, to do a 64-bit MIPS Chrome OS cross-compile:
 
 ```
 gn gen out/Default --args='target_os="chromeos" target_cpu="mips64el"'
 ```
 
 ## As a BUILD.gn author
 
 If you are editing build files outside of the //build directory (i.e.,
 not directly working on toolchains, compiler configs, etc.), generally
 you only need to worry about a few things:
@@ skipping 55 matching lines @@
 `target_os` and `target_cpu`. The `current_toolchain` reflects the
 toolchain that is currently in effect for a rule.
 
 Be sure you understand the differences between `host_cpu`, `target_cpu`,
 `current_cpu`, and `toolchain_cpu` (and the os equivalents). The first
 two are set as described above. You are responsible for making sure that
 `current_cpu` is set appropriately in your toolchain definitions; if you
 are using the stock templates like `gcc_toolchain` and `msvc_toolchain`,
 that means you are responsible for making sure that `toolchain_cpu` and
 `toolchain_os` are set as appropriate in the template invocations.