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PassthroughProgramCache is effectively a singleton. https://cs.chromium.org/chromium/src/gpu/command_buffer/service/passthrough_program_cache.cc?rcl=9f6874260539e70ec774687f49a56d6fd1feb8f5&l=49

We need to grab it from GpuChannelManager to support shader caching
using Angle.

TBR=boliu@chromium.org

Change-Id: I43378d11f01bcc0bac37faa228f72db404214464
Reviewed-on: https://chromium-review.googlesource.com/c/1427221
Commit-Queue: Jonathan Backer <backer@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625316}