GitLab

This splits part of chrome_render_frame.mojom into
render_frame_prerender_messages.mojom

and creates a class PrerenderRenderFrameObserver to handle
the StartPrerendering mojom message.

The motivation is to remove chrome deps from PrerenderContents so it can
be componentized.

Bug: 1096088
Change-Id: I60146c2af867e7b57de392c97e010dc481338d06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2324842
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792651}