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There is logic in the ClientSession/ClientContext which attempts
to drop a connection on the IO thread. Unfortunately, in doing so,
it called a thread-unsafe member getter within the call to PostTask.

This CL moves the getter into a helper method which runs entirely
on the IO thread.

R=nyquist@chromium.org,wez@chromium.org
CC=steimel@chromium.org
BUG=647814

Review-Url: https://codereview.chromium.org/2344893009
Cr-Commit-Position: refs/heads/master@{#419491}