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Make Files, Gallery and Audio apps work with HTML Imports polyfill.
The tests aren't passing yet, which I'm fixing on CL:1462640, but I
have tested these 3 apps manually locally with:

    --disable-blink-features=HTMLImports

and they seem to be working. However, I haven't tested them heavily,
but I did test things related to Polymer elements which are affected
by HTML imports.

Add connect-src chrome://resources to Audio and Gallery's manifests
(content_security_policy) to allow HTML import polyfill to URL fetch
resources (via xmlHTTPRequest). File Manager's manifest already had
this CSP setting.

Bug: 924873
Change-Id: I43dc5962be9d8cbdcf795b35b8de909578ac4279
Reviewed-on: https://chromium-review.googlesource.com/c/1480296Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634521}