Make StyleChange parameter explicit in setNeedsStyleRecalc.

The SubtreeStyleChange is a very expensive change, yet the default parameter
used for changeType in setNeedsStyleRecalc. It is made non-default here in
order to enforce a more conscious choice when adding calls to
setNeedsStyleRecalc.

BUG=None

Review URL: https://codereview.chromium.org/152623002

git-svn-id: svn://svn.chromium.org/blink/trunk@166335 bbb929c8-8fbe-4397-9dbb-9b2b20218538