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Add .mjs to type mappings for the javascript MIME type. This is
necessary for .mjs to be recognized as a valid for use in modules
when served by the extension protocols, where the mime type is
determined by our net code (as opposed to a server).

Update the extension API test for modules to include a .mjs file test.

Bug: 857076
Change-Id: Ib14cabbc52057cfb57c933de5a9c3ed4b13b87d2
Reviewed-on: https://chromium-review.googlesource.com/1117203Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571921}