Revert "Allow SequencedTaskRunnerHandle::Get() while running unsequenced tasks."

This is effectively a manual revert of 5d315e4b4a7cdef34b0c1e863353d03a30781217.

Per the outcome of the discussion on http://crbug.com/618043 and associated doc/chromium-dev discussion.

BUG=618043

Review-Url: https://codereview.chromium.org/2177373005
Cr-Commit-Position: refs/heads/master@{#408440}
6 files changed