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Previously, the HintCache's initialization, which involved adding all
hints contained within a hint vector to the cache, was being handled
on the UI thread. This initialization could take a significant amount
of time and triggered the creation of a jank startup performance bug.

The logic has been modified so that the HintCache is no longer
initialized on the UI thread. It is instead initialized within its
constructor during PreviewsHints::CreateFromHintsComponent() on a
background thread.

This is accomplished via a HintCache::Data object. Once the hints are
fully added to Data, it is moved into the HintCache's constructor. The
HintCache is immutable after construction.

Not only does this remove all of the costly UI thread processing, but
it also eliminates the expensive intermediate hint vector.

Bug: 910251
Change-Id: I04a28248ad1ce790f999da06f48e3c5aabfcb812
Reviewed-on: https://chromium-review.googlesource.com/c/1356226
Commit-Queue: Jered Gray <jegray@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613196}