Explicitly set gcc as it's derived in gyp.

Bug: chromium:645890
Change-Id: Ibe77ab10b9ce705bdb650b14e212c55b0a9154ba
Reviewed-on: https://chromium-review.googlesource.com/552638Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Zhengxing Li <zhengxing.li@intel.com>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46295}