Clear StyleResolver state before returning from styleForElement().

      A cached element pointer in the resolver state was causing confusion because
in some cases a subsequent call to styleForElement() would use a pointer to
a different object that is at the same memory address as the previous one.

R=dglazkov@chromium.org
BUG=196393

Review URL: https://codereview.chromium.org/14064008

Patch from Ken Buchanan <kenrb@chromium.org>.

git-svn-id: svn://svn.chromium.org/blink/trunk@148687 bbb929c8-8fbe-4397-9dbb-9b2b20218538