Fix Clang build on Android.

This is part of Chrome for Android upstreaming.

In case we are doing a Clang build, we have to unset CC_target and CXX_target
(if they are gcc-related).
Otherwise GYP ends up generating a gcc build (although we set 'clang' to 1 in
$GYP_DEFINES).
This behavior was introduced by 54d2f6fe6d8a7b9d9786bd1f8540df6b4f46b83f in
GYP.

BUG=143889


Review URL: https://chromiumcodereview.appspot.com/10918045

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@155185 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed