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This CL adds a new GLContextVirtualDelegate interface to contain all
context state related methods from DecoderContext. After this one, I am
going to implement GLContextVirtualDelegate in RasterDecoderContextState
and only create one GLContextVirtual for all raster decoder and display
compositor.

Bug: 900941, 902904
Change-Id: I10d80bea8531885c449a2e676ce0552e9363b34c
Reviewed-on: https://chromium-review.googlesource.com/c/1312338Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606777}