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This CL removes all the dependencies on ContentViewCore in native
world. Rather than WebContentsViewAndroid owning it, ContentViewCore
owns and deletes itself by way of a WebContentsObserver notification.
Also removes a few methods that are not in use any more.

Bug: 598880
Change-Id: Ib8bcfdf3cb39332ea3f0f3186c3692ce67d82704
Reviewed-on: https://chromium-review.googlesource.com/999455
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550008}