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A lot of the metrics that were being used around capture for NTP aren't
actually relevant for thumbnailing for tab hover/switching page
previews. This CL eliminates the extra logic and background work around
thumbnail quality as well as simplifying several of the classes and
utilities involved.

This CL does not:
 - correct the logic for when thumbnails should be captured
   (it is still tuned for the NTP use case)

This will be done in a followup CL.

Bug: 928954
Change-Id: I05c8f5c6a2f276419c72302892c7fe7cb9fa334a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529853
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642443}