Eliminate flakiness for inspector/styles/pseudo-elements.html

Explicitly wait for the elements tree to update.

R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/25693002

git-svn-id: svn://svn.chromium.org/blink/trunk@158711 bbb929c8-8fbe-4397-9dbb-9b2b20218538