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Hiding the async_waiter in a static makes it difficult to deal with
different threads running potentially different loop types running in
the same process. Making this explicit in the create call is much easier.
This assumes that callers will want to permantently bind a context with
a particular async waiter. In the case that we end up with a caller that
wishes to create a context on a thread using one async waiter type and
then bind that context to a thread using a different async waiter type
we can add another version of MakeCurrent, but that seems like a fairly
remote possibility at this point.

R=piman@chromium.org

Review URL: https://codereview.chromium.org/428413002

Cr-Commit-Position: refs/heads/master@{#289074}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289074 0039d316-1c4b-4281-b951-d872f2087c98