[ MimeHandlerView ] Runtime Feature for blink

Frame-based MimeHandlerView is currently behind a flag which is defined
in content features. This CL adds a corresponding runtime feature flag
which is drived by the content feature. This is needed in upcoming CLs
for blink changes needed to support frame-based MHV.

The proof of concept CL that benefits from this flag is: issue 1287198

Bug: 659750
Change-Id: Ia127f0cd01b7123ff4a592a7334d90dd417f0d6a
Reviewed-on: https://chromium-review.googlesource.com/c/1459336
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632234}
6 files changed