Add option cc_wrapper to GN

docs/ccache_mac.md

 # Using CCache on Mac
 
 [ccache](http://ccache.samba.org/) is a compiler cache. It speeds up
 recompilation of C/C++ code by caching previous compilations and detecting when
 the same compilation is being done again. This often results in a significant
 speedup in common compilations, especially when switching between branches. This
 page is about using ccache on Mac with clang and the NinjaBuild system.
 
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 ```
 
 (Instead of relying on the clang/clang++ for building chromium in your `$PATH`,
 you can also use the absolute path here.)
 
 ## Use with GN
 
 You just need to set the use\_ccache variable. Do so like the following:
 
 ```shell
-gn gen out-gn --args='use_ccache=true'
+gn gen out-gn --args='cc_wrapper="ccache"'
 ```
 
 ## Build
 
 In the build phase, the following environment variables must be set (assuming
 you are in `chromium/src`):
 
 ```shell
 export CCACHE_CPP2=yes
 export CCACHE_SLOPPINESS=time_macros
 export PATH=`pwd`/third_party/llvm-build/Release+Asserts/bin:$PATH
 ```
 
 Then you can just run ninja as normal:
 
 ```shell
 ninja -C out/Release chrome
 ```
 
 ## Optional Steps
 
 *   Configure ccache to use a different cache size with `ccache -M <max size>`.
     You can see a list of configuration options by calling ccache alone.  * The
     default ccache directory is `~/.ccache`. You might want to symlink it to
     another directory (for example, when using FileVault for your home
     directory).