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Replace the GLInProcessContextSharedMemoryLimits class with our new
SharedMemoryLimits class, so we can share implementations and defaults.

R=boliu@chromium.org, piman@chromium.org
BUG=584497
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1898033005

Cr-Commit-Position: refs/heads/master@{#388633}