This simplifies the rolling process since the build configuration was
fairly complex. It allows us to save on maintaining the two sets of
build files. The back-end is also disabled in GYP.

The back-end sources are kept in libGLES.gypi for now to keep all
libANGLE sources in the same place.

Bug: angleproject:2237, angleproject:1569
Change-Id: Ie73b40ef125cfc20da6f4ed3b33339256f1c5172
Reviewed-on: https://chromium-review.googlesource.com/805416
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>