Update extensions::MessagingBindings to use WebScopedWindowFocusAllowedIndicator with WebDocument.
This will allow Blink to delete some deprecated code.
This is part of a three-sided CL:
Part 1: https://codereview.chromium.org/
777483004/
Part 2: <this>
Part 3: https://codereview.chromium.org/
722423004
BUG=440740
Committed: https://crrev.com/
ebb4bf8090b06979fd44411af69304b505681f4d
Cr-Commit-Position: refs/heads/master@{#307914}
Review URL: https://codereview.chromium.org/
786403003
Cr-Commit-Position: refs/heads/master@{#326007}