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This is largely a copy-and-paste from GLES2DecoderImpl, with some minor
cleanup.

- added a vertex array manager to RasterDecoderImpl and copied
  associated boilerplate because it was required by
  CopyTextureCHROMIUMResourceManager

- copied GLES2DecoderImpl::CopySubTextureHelper as
  RasterDecoderImpl::CopySubTexture with very minor modification
  (e.g. raster is given a destination texture ID instead of the target
  that GLES2 takes)

- added a unittest to verify that source and destination textures are
  cleared appropriately

Bug: 789238
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I1eb50e43cc52ee9931a483ab89f9cdbdf47e2332
Reviewed-on: https://chromium-review.googlesource.com/993763
Commit-Queue: Jonathan Backer <backer@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548258}