CanvasResourceGpuMemoryBuffer: remove |image_id_| member

This CL follows crrev.com/c/1056168 and removes |image_id_| from
CanvasResourceGpuMemoryBuffer, since it's not needed (it was
mentioned in that CL description but finally not removed).

Bug: 839970, 903837
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I7540363f131e3a33609c945e68c8106992e0ba77
Reviewed-on: https://chromium-review.googlesource.com/c/1130170
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606927}
4 files changed