Get rid of the StyleResolver throwaway timer.

Since we moved RuleSets to live off StyleSheetContents, this memory
optimization really doesn't buy us anything. In the future, we might
want to consider clearing the RuleSets of the StyleSheetContents,
but in the meantime, we don't need this code.

Review URL: https://codereview.chromium.org/139153013

git-svn-id: svn://svn.chromium.org/blink/trunk@165355 bbb929c8-8fbe-4397-9dbb-9b2b20218538