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1. StreamTextureHost's |channel_| could be null while
StreamTextureHost is still alive because |channel_| can be
set to null on a OnChannelError() call.

2. Add a new unit test to test and make sure StreamTextureHost
can handle a null |channel_|.

3. An invalid mailbox needs to be checked by calling
Mailbox::IsZero().

4. Remove previously added CHECK's for debugging.

Bug: 984309
Change-Id: I1fb42855877540f1ec0ddbcec4fe5713071727a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1740168
Commit-Queue: vikas soni <vikassoni@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686642}