DevTools: [Media Inspector] Support inspected page scale.

This patch adds support for inspected page scale. It should be landed
after the full-featured support of the inspected page scale.

Review URL: https://codereview.chromium.org/332423003

git-svn-id: svn://svn.chromium.org/blink/trunk@176714 bbb929c8-8fbe-4397-9dbb-9b2b20218538