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This doesn't handle all the cases (e.g. plugins, downloads) but these don't work
yet with the network service. However it does fix 900 new wpt tests that 
started failing after r487304 since these generated *worker.html files weren't
being served with a mime type which was causing their load to fail.

BUG=746144,729849

Change-Id: I232c9af1dca32f1c641736c8ccd61978096559ef
Reviewed-on: https://chromium-review.googlesource.com/577507Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488445}