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/337708Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>