Don't clear pending updates inside styleForRenderer()

We only ever call styleForRenderer() from inside recalcStyle now,
there's even an ASSERT about it so there's no reason to clear the
pending as there never will be one anyway.

Review URL: https://codereview.chromium.org/267933004

git-svn-id: svn://svn.chromium.org/blink/trunk@173283 bbb929c8-8fbe-4397-9dbb-9b2b20218538