Disable single channel renderability for ANGLE ES2

Bug: 720325

Change-Id: Ib55c7a0c59a23b8dc012e04b626010de7da2921b
Reviewed-on: https://skia-review.googlesource.com/18485
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed