Build GN with c++11.

GN code does not need GNU extensions to be built like some other
Chromium code.

BUG=427584
TEST=./tools/gn/bootstrap/bootstrap.py, the manual part still works.
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/1358243003

Cr-Commit-Position: refs/heads/master@{#350558}
1 file changed