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This change teases apart the code that runs in a renderer that observes
link tags from the code that runs in a renderer used to perform no-state
prefetching. That translates to moving the PrefetchFinished signal from
PrerenderDispatcher to PrerenderHelper.

This change also goes one step further and, recognizing that
PrerenderHelper is a RenderFrameObserver, ditches the PrefetchFinished
method on WebPrerenderingSupport in favor of just observing
DidFinishDocumentLoad (aka DOMContentLoaded). The timing of the two
signals is approximately the same and for what the no-state prefetching
code requires, this seems sufficient.

URLLoaderThrottleProvider is simplified by moving most of the logic
associated with creating the PrerenderURLLoaderThrottle moved into
PrerenderHelper, further consolidating logic around the throttles (again
used only by the renderer process being used to drive the no-state
prefetch).

There's a slight change to the calculation of the start time for the
parse time histogram. It should not be material, and if anything, this
should be a less errorprone way of calculating it. Having it align to
the creation of the PrerenderHelper means that we aren't reliant on
prefetch happening in a newly created process to have a properly
computed start time. Hopefully this change does not goof up any metrics.

This CL sets the stage for onion-souping the renderer code used to
observe link tags, kick-off prefetches and observe their activity. We
should be able to eliminate WebPrerenderingSupport and WebPrerender,
for example.

Change-Id: Ia337c74484d8ff9fd8e1fdb57f0a3e282a03b8df
Bug: 967834
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032424
Commit-Queue: Darin Fisher <darin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737559}