[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.

Bug: 28814913
Test: Android-on-Chrome OS has the same behavior as before
Test: Android-on-Chrome OS, when setting the DefaultExceptionHandler's
      delegate can now forward the unhandled exceptions to the crash
      server.
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}
4 files changed