Gyp support for Chrome OS. Change comment to explain the 'chromeos' gyp flag. Make it possible to override pkg-config. BUG=angleproject:1297 Change-Id: Ifc56cc4575d83ff893aaa0283c3aaa91df14fafd Reviewed-on: https://chromium-review.googlesource.com/337708 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Frank Henigman <fjhenigman@chromium.org>