Remove RenderTheme::hasOwnDisabledStateHandlingFor().

RenderThemeChromiumSkia and RenderThemeChromiumMac always return true, which means we can eliminate a lot of dead code in MediaControls that had to handle the false case.

As a result MediaControls::reportedError() can be removed entirely since it no longer does anything and MediaControls::loadedMetadata() can be replaced with MediaControls::reset().

BUG=337015

Review URL: https://codereview.chromium.org/146063002

git-svn-id: svn://svn.chromium.org/blink/trunk@165684 bbb929c8-8fbe-4397-9dbb-9b2b20218538