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Before this CL, URLFetcher could only be instantiated from a
single-threaded task. The response was dispatched on the origin thread.

With this CL, URLFetcher can also be instantiated from a sequenced
task. The response is dispatched on the origin sequence.

Since SequencedTaskRunnerHandle behaves the same way as
ThreadTaskRunnerHandle when used from a single-threaded task, this
CL does not affect the behavior of existing URLFetcher call sites
(they are necessarily in single-threaded tasks because
base::ThreadTaskRunnerHandle::Get() crashes when called from
a non-single-threaded task).

BUG=675631

Review-Url: https://codereview.chromium.org/2599873002
Cr-Commit-Position: refs/heads/master@{#443224}