GL: Fix the CPU readback path of CopyTextureCHROMIUM. 1. BlitGL's resources were not being intialized. 2. The format/type information was not enough to know if the destination texture was SRGB. BUG=693090 Change-Id: I3fc277a175772d3b6acace1810cb43f4a9bdc473 Reviewed-on: https://chromium-review.googlesource.com/982642 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>