Use LocalStyleChange for updating :-webkit-drag.

If you don't have childrenAffectedByDrag, use LocalStyleChange instead of
SubtreeStyleChange to avoid unnecessary recalc of the whole subtree.

R=ojan
BUG=341831

Review URL: https://codereview.chromium.org/140053005

git-svn-id: svn://svn.chromium.org/blink/trunk@166746 bbb929c8-8fbe-4397-9dbb-9b2b20218538