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The call to clear() was accidentally removed in https://crrev.com/456456,
but the bug didn't surface until recently when the remote plugin list
was updated before the baked-in one.

Other cleanup:
* Remove unused FindPlugin() method
* Use built-in static instead of base::Singleton
* Use SequenceChecker instead of BrowserThread thread checking
* Use new base::Value APIs and moar std::unique_ptr

TBR: finnur@chromium.org
Bug: 778856
Change-Id: I050dd2292882ab047b54f63983cdfd2f7da3aded
Reviewed-on: https://chromium-review.googlesource.com/746841
Commit-Queue: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512853}