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This is for reuse by RasterDecoder in a subsequent CL. The test added
for GLES2DecoderImpl is similar to what will be added for
RasterDecoderImpl.

Changes:
- made CopyTextureMethod an enum class for easy forward decl
- moved ValidateCopyTexFormatHelper and GetCopyTextureCHROMIUMMethod for
  future reuse
- added mock for texture copying
- added unittest to make sure that texture copying clears source and
  destination textures 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: I44a1ca37ad48d065acfd9d94b737ac1d925d99e5
Reviewed-on: https://chromium-review.googlesource.com/993592
Commit-Queue: Jonathan Backer <backer@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548228}