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This CL converts PrerenderMsg_SetIsPrerendering to
ChromeRenderFrame Mojo message, and pass the message to
chromePluginPlaceholder from ChromeRenderFrameObserver via
PrerenderObserver.

Bug: 798712
Change-Id: Id84240d8f16fc811df56fc05a734a4ee539c2a91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2246015Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Darin Fisher <darin@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/master@{#782849}