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As pointed out in the bug, unhandled errors on worker threads were not
properly being propagated for the parent thread to handle as the spec
requires (https://html.spec.whatwg.org/multipage/workers.html#runtime-script-errors-2).
This fix adds in the code to do that.

Bug: 685303
Change-Id: Ie3e7ec03153a43efdc6e70b874d6b1a1d8ac35ef
Reviewed-on: https://chromium-review.googlesource.com/c/1287208
Commit-Queue: Katie Dillon <kdillon@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605857}