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This adds PrerenderContentsDelegateImpl, PrerenderManagerDelegateImpl
and PrerenderManagerFactory.

A simple browsertest has been added to verify functionality.

Bug: 1096088
Change-Id: I8682eecc2deb10ac5da77d0892a7abf544aa839d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339463Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796225}