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There are two JS hooks we used in handling exceptions in extensions
bindings:
- There is an exposed "handleException" method, which handles exceptions
  encountered in running untrusted code.
- The test API allows us to set custom handlers for uncaught exceptions
  (so we can fail a test or verify expected errors).

Implement each of these in the utilities exposed to the JS bindings, and
add unittests for the same.

Bug: 653596
Change-Id: I246308c78b0760f5176949c2b1d88da07dca337a
Reviewed-on: https://chromium-review.googlesource.com/568692Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486667}