Revert of Don't clear pending updates inside styleForRenderer() (https://codereview.chromium.org/267933004/)

Reason for revert:
I'm not sure this patch is correct, lets revert it until I understand the implications better.

Original issue's description:
> 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.
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173283

TBR=shans@chromium.org,dstockwell@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/268973002

git-svn-id: svn://svn.chromium.org/blink/trunk@173284 bbb929c8-8fbe-4397-9dbb-9b2b20218538