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This change exposes a Java method on WebContentsObserverProxy that
allows it to observe render frame creation.
Follow-up CLs will inject objects in the RenderFrameCreated
handler as part of the mojofied Java Bridge browser integration.

Bug: 794320
Change-Id: Ied51a88887e9d53744f253ce7d817d126557b608
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274328Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784161}