Add a flag to GrSurfaceFlags that requires the texture to be cleared upon creation. 

Bug: chromium:656320

Change-Id: I940bfa24540516ab83a2ed52f761b96eb6ad19f1
Reviewed-on: https://skia-review.googlesource.com/17391
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
14 files changed