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This CL converts WebCachePtr and WebCacheRequest
to new Mojo types using Remote, ReceiverSet and
PendingReceiver.

Bug: 955171
Change-Id: Ie26db230a04880aca1fe6be076ae43ae01dfc492
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1818881Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Julie Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#699612}