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The logic in PreviewsHints::CreateFromConfig() has been modified so that
it now strips out optimizations that are disabled and only allows one of
each optimization type within a PageHint. Duplicate types encountered
are also stripped. This makes it so that enabling an experimental
optimization will automatically disable a subsequent optimization of the
same optimization type in the same PageHint. If all optimizations for a
page hint are stripped, then the page hint is also stripped.

This fixes a bug related to disabled page hint optimizations being
usable, as the disabled optimizations will no longer exist in the
hint cache.

As a result of this change, when the total page hints with resource
hints exceeds the cap, the number of page hints added will always
exactly match the cap. This is due to the logic for stripping hints
allowing us to add hints that only include some of their page hints.

A DCHECK has been added verifying that non-resource loading hint
optimizations never contain resource loading hints.

Additionally, the description of |whitelisted_optimizations| for the
PageHint proto has been updated to match expectations that it will be
provided as an ordered list.

Unittests have been added to verify that the stripping is handled
properly and that the number of page hints allowed stops exactly at the
cap. Some issues with unittests in PreviewsOptimizationGuideUnittest
using InitializeMultipleResourceLoadingHints() have also been fixed.

Bug: 900725, 900733
Change-Id: Ic8d8ce2de98e4e2993cab994906c3b2e43c31a22
Reviewed-on: https://chromium-review.googlesource.com/c/1311796Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Jered Gray <jegray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604773}