Remove some unecessary code from pseudoStyleRulesForElement.

-The null check really belongs in getMatchedCSSRules as that's the
only codepath that might pass a null.
-Resetting documentElement state is not correct here. We already
do the resetting in all the appropriate styleFor* methods. This method
is called from editing and the inspector. It shouldn't be messing
with document state.

Review URL: https://codereview.chromium.org/53103002

git-svn-id: svn://svn.chromium.org/blink/trunk@160962 bbb929c8-8fbe-4397-9dbb-9b2b20218538