Mojo: Implement plumbing to support passing handles over MessagePipes.

This is tricky for several reasons:
  - We have fine-grained locking (and would like to keep it that way),
    and need to avoid deadlock. In particular, acquiring multiple
    dispatcher locks simultaneously is dangerous -- so we allow it to
    fail.
  - We want clean failure semantics. In particular, on failure,
    WriteMessage() should leave the handles being sent valid. This means
    that we may not remove them from the handle table until the call has
    succeeded. Thus we have to mark them as busy.
  - We need to avoid various races. E.g., still to do: When sending a
    handle in-process, it's important that once |WriteMessage()| has
    sent a handle, no more calls done on that particular handle may
    proceed. As a result, we won't be able to simply transfer
    dispatchers to a new handle (in-process) but instead must create a
    new dispatcher referencing the same resource. This will also ensure
    that |Wait()|s on that handle will be properly cancelled.

R=darin@chromium.org, darin

Review URL: https://codereview.chromium.org/67413003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234302 0039d316-1c4b-4281-b951-d872f2087c98

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