DevTools: Persist Overrides settings across DevTools restarts

Override settings are now persisted, and we have two new checkboxes:
* "Enable" - toggles all checked overrides impact on the inspected page
* "Enable on DevTools startup" - controls the "Enable" checkbox state
  when DevTools are started and automatically checks it once enabled.

R=pfeldman, vsevik
BUG=175966

Review URL: https://chromiumcodereview.appspot.com/18210005

git-svn-id: svn://svn.chromium.org/blink/trunk@154638 bbb929c8-8fbe-4397-9dbb-9b2b20218538