[mojo] Add a way to handle unhandled RuntimeExceptions

This change makes it possible to allow interfaces to globally handle
unhandled RuntimeExceptions, in their bindings or in the callbacks.

      delegate can now forward the unhandled exceptions to the crash
      server.

Bug: 810087
Test: Android-on-Chrome OS has the same behavior as before
Test: Android-on-Chrome OS, when setting the DefaultExceptionHandler's
Change-Id: I2b7455a0344a109e1d2416a74ad4a0b98cd007f0
Reviewed-on: https://chromium-review.googlesource.com/1101898


Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Luis Hector Chavez <lhchavez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568128}