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The previous logic in OptimizationGuideService created an
optimization_guide::proto::Configuration object on a background thread
and then used task posting to send it on to
PreviewsHints::CreateFromConfig(), where it was used to create a
PreviewsHints object on another background thread. In all, there were
two tasks posted that included the config object as a parameter, one on
the background thread and one on the UI thread. Including it as a
parameter in a task requires a full copy of the protobuffer. This means
that a full copy of the config protobuf, which can be as large as 1.8MB
(the size of the current Brazil one on Canary), was occurring on the UI
thread.

In local performance measurements, making a single copy of a 600KB
version of the protobuf took several milliseconds (it amounted to
roughly 60% of the time taken by the initial component string parsing
and 60% of the time taken by PreviewHints::CreateFromConfig()). Given
that we were incurring the cost of two copies, one of which was on an
extremely high priority thread, it makes sense to change the logic to
eliminate the need for the copies.

The logic has been altered so that OptimizationGuideService no longer
immediately processes the component, but instead notifies the
observers that it is available and allows them to trigger the
processing. This eliminates both copies of the configuration protobuf,
as it is now created where it is used.

Additionally, OptimizationGuideServiceObservers are now immediately
notified of the hints component when they register if one is already
available. This will enable the PreviewsOptimizationGuide to wait until
the HintCacheLevelDBStore is fully initialized before registering for
the component, and in the future will potentially allow it to avoid
processing the configuration altogether when the store already has the
latest version cached.

New unittests have been added and older ones have been updated to
accommodate the new logic.

The related browser tests have also been modified to be more robust,
where they now wait for a signal from a local histogram indicating
that hint processing is complete.

Bug: 908985

Change-Id: I330fcc35f14388b8a704d3180bd758e7327e9892
Reviewed-on: https://chromium-review.googlesource.com/c/1351719
Commit-Queue: Jered Gray <jegray@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611982}