DevTools: fix resource mimetype for request interception of file:// urls

This patch starts fetching mime type from the subrequest, if any.

R=caseq

Change-Id: Ic3ca487fa7ee80f5d8a1bcc270f7d7120fee8fc5
Reviewed-on: https://chromium-review.googlesource.com/1010415
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550319}
4 files changed