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This reverts commit 1b420fdb.

Reason for revert: Makes PreviewsNoScriptBrowserTest and 
ResourceLoadingHintsBrowserTest flaky

https://crbug.com/909998
https://crbug.com/909999


Original change's description:
> Eliminate unnecessary proto::Configuration copies
> 
> 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}

TBR=waffles@chromium.org,tbansal@chromium.org,dougarnett@chromium.org,ryansturm@chromium.org,jegray@chromium.org

Change-Id: I65e2ffff8737cc89bcc0726b4c3e67271e654c2d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 908985
Reviewed-on: https://chromium-review.googlesource.com/c/1354745Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612044}