Use mojo to communicate between Javascript and C++.

Displays basic previews status on chrome://interventions-internals. Right now it
is only displaying Offline Previews' status. Functionalities and tests for C++ code will be
in future CLs.

Added tests for the basic JavaScript method for displaying previews'
enabled/disabled status.

Bug: 764413
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Id943304b28cd6fb64f0e1ed72a12e392d7cfda8e
Reviewed-on: https://chromium-review.googlesource.com/671499
Reviewed-by: Mustafa Emre Acer <[email protected]>
Reviewed-by: Demetrios Papadopoulos <[email protected]>
Reviewed-by: Ryan Sturm <[email protected]>
Commit-Queue: Thanh Le <[email protected]>
Cr-Commit-Position: refs/heads/master@{#503862}
15 files changed