Loosen thread-safety checks and update documentation on RunLoop.
RunLoop::Delegate now being the thread-affine part. RunLoop itself is now
merely thread-unsafe (->sequence checks).
And make it safe to access a RunLoop's state from another sequence while it's
being Run() as that access is technically "sequenced" (any access will rebind
the SequenceChecker to that sequence for the remainder of the run and should
still catch undesired concurrent accesses).
The lack of detach during Run() might also be the cause of issue 715235
(will try to remove TODOs after this lands).
BUG=722537, 715235
Review-Url: https://codereview.chromium.org/2886913003
Cr-Commit-Position: refs/heads/master@{#472835}
2 files changed
