Content feature for MimeHandlerView in OOPF

This CL introduces a bare content feature to allow landing experimental
work and future CLs on MimeHandlerView using cross process frames. The
feature will be disabled by default.

MimeHandlerViewContainer is currently the last remaining GuestView
which still relies on BrowserPlugin. Eventually and starting with this
CL, the new implementation of MimeHandlerView will be based on
cross-process frames that no longer requires a BrowserPlugin.

Bug: 659750, 330264
Change-Id: I52a8eba4e9f2446ae0d899cd3a639f418bc01d12
Reviewed-on: https://chromium-review.googlesource.com/1073923
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580240}
2 files changed