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There are a series of messages that are sent to the RenderThreadObserver
of the webview. This maps ClearCache onto one of those messages.
Other targeted messages will be converted in followup CLs.

BUG=993189

Change-Id: I93d84f08bfcc74211ac175463754bf6b9d3d7ce8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515043
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824874}