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With the new task scheduling system, there are no guarantees that
different operations will not collide - each caller has to ensure that
no conflicting tasks will be scheduled and executed synchronously. This
means that for file tasks, we need to have a common task runner for any
tasks that may touch the same files.

Introduce a common SequencedTaskRunner for the extension system, and use
it in the UserScriptLoader.  Additionally, use it in ExtensionService,
in lieu of the prior ExtensionService-owned task runner that's used for
file tasks in installation.  Finally, also make the GetFileTaskRunner()
method in ExtensionService non-virtual, since it's not needed on any of
the interfaces.

Bug: 689520
Bug: 750122
Change-Id: I9d0b993689821e7aca9b2553283609cce796a162
Reviewed-on: https://chromium-review.googlesource.com/593854Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491209}