Corrections to GLES sRGB formats. This stops Mesa from complaining when using GLES 3.0 in the back end. BUG=angleproject:1301 Change-Id: Ib9c0397586a11c407c6aa9ad1544e12b43dd08a7 Reviewed-on: https://chromium-review.googlesource.com/324031 Tryjob-Request: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>