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ChannelDelegate::GetWeakPtr() was called from the UI thread but
it was racy as the weak ptr is also used in the IO thread.
This CL turns ChannelDelegate a ThreadSafeRefCounted so that
we can pass ChannelDelegate itself to the task runner, instead
of using its weak ptr on the UI thread.

This change also turns some TaskRunner declarations to
SequencedTaskRunner to access its DeleteSoon() API from
ChannelDelegate.

TBR=creis@chromium.org
R=viettrungluu@chrormium.org, agl@chromium.org
BUG=460243

Review URL: https://codereview.chromium.org/955813002

Cr-Commit-Position: refs/heads/master@{#318994}