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This adds various entrypoints to Prerender code in WebLayer, now that
all the delegate interfaces have implementations and all factories are
in place.

Please see the following spreadsheet to see corresponding chrome
entrypoints and my reasoning for the chrome entrypoints I have left out
of WebLayer:
https://docs.google.com/spreadsheets/d/1u13bzBQiBqibXzDDmkB4Kbvm29DslTQmXmY91864gOU/edit#gid=299677236

Bug: 1096088
Change-Id: I62549bf3a8f6dbd5a5334c87048b197d75fc4001
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346349
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Martin Barbella <mbarbella@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797753}