Currently, the gyp variables CC_wrapper and CXX_wrapper can be set in
the chromium.gyp_env file. However, the host compiler wrapper variable
cannot. By exposing this variable, embedders can more easily
use tools like ccache or icecream when developing for android.

BUG=471340

Review URL: https://codereview.chromium.org/1044423005

Cr-Commit-Position: refs/heads/master@{#323557}