DevTools: Rename UserAgentSupport to a more general OverridesSupport

UserAgentSupport does much more than managing the emulated User-Agent.
Renaming the class to reflect this.

R=pfeldman, vsevik
BUG=175966

Review URL: https://chromiumcodereview.appspot.com/18690002

git-svn-id: svn://svn.chromium.org/blink/trunk@153615 bbb929c8-8fbe-4397-9dbb-9b2b20218538