DevTools: [Media Inspector] Drastically reduce amount of re-renderings

Media Inspector polls media queries from backend whenever it suggests
that something might change. Currently, every polling results in
re-rendering of media inspector even though the media queries set
did not change. This results in scrolling lags.

This patch does rerendering on new media queries iff they actually
changed.

R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/344193002

git-svn-id: svn://svn.chromium.org/blink/trunk@176615 bbb929c8-8fbe-4397-9dbb-9b2b20218538