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If we recieve a request for an extension that doesn't have a 'cast_url'
key present defer to the normal extension request processor to load
the files from disk

This is also required to load background pages (as it is from disk)

Bug: b/73768170
Test: Load extension that loads from disk, verify fallback occurs
Change-Id: I7bf477f92dbebfd37077ed417e12d17a03d35308
Reviewed-on: https://chromium-review.googlesource.com/934924Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Albert Chaulk <achaulk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553682}