Any projects that may be included by gyp files outside our project (chrome
or skia) may not have the same defines or ignored compiler warnings.  To
make sure that we can always compile, each project now includes a common
file with all required definitions and gyp variables.

BUG=angleproject:583

Change-Id: I702bee975d0554c51bfa03981920dfb295ffbafa
Reviewed-on: https://chromium-review.googlesource.com/189458Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>