Labelling this with M-57, please adjust the milestone if not proper.

Thank you!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0c3ac0678cbc511f793dfd7386729b1b2ae2e7e1

commit 0c3ac0678cbc511f793dfd7386729b1b2ae2e7e1
Author: watk <watk@chromium.org>
Date: Fri Mar 31 03:37:02 2017

mojo: Support sync calls through ThreadSafeInterfacePtr

Previously ThreadSafeInterfacePtr only supported async calls. Now it
also supports sync calls.

The one caveat to be aware of is that sync calls will block both the
calling thread and the thread that the underlying InterfacePtr is bound
to. That means, e.g., that the InterfacePtr can't be bound to the IO
thread. Letting the call be async on the InterfacePtr thread is left as
a TODO.

BUG= 668565 

Review-Url: https://codereview.chromium.org/2770153003
Cr-Commit-Position: refs/heads/master@{#461016}

[modify] https://crrev.com/0c3ac0678cbc511f793dfd7386729b1b2ae2e7e1/mojo/public/cpp/bindings/tests/sync_method_unittest.cc
[modify] https://crrev.com/0c3ac0678cbc511f793dfd7386729b1b2ae2e7e1/mojo/public/cpp/bindings/thread_safe_interface_ptr.h

This is done. I filed a new bug to address the final TODO: http://crbug.com/714807