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The main consumer of DistillabilityResult on desktop, the Reader Mode
icon, updates whenever the user switches tabs. However, the
distillability service usually will not run again when the tabs are
switched, so the icon will not update if it sees the web contents'
distillability only when OnResult() is called. It needs to be retrieve
the distillability of a given web contents in between calls to
OnResult(), as well as keep track of which web contents the result
belonged to.

The simplest solution is to store the result in DistillabilityDriver,
since it is already correctly scoped to a single web contents.

Bug: 952042
Change-Id: I8a8633a52583f04b49298ee5186b24a5eb092fdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1788498Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#697724}