Don't synchronize attributes unless necessary when matching
attribute CSS selectors.

Previously, synchronization was always happening, causing a
large performance impact due to syncing of inline styles
into the "style" attribute on some pages that updated
styles on every frame.

Fix another small bug in which the two versions of synchronizeAttribute in the Element class were not functionally equivalent (in particular, with regard to case-sensitive matching on the style attribute).

BUG=322239

Review URL: https://codereview.chromium.org/100433003

git-svn-id: svn://svn.chromium.org/blink/trunk@163464 bbb929c8-8fbe-4397-9dbb-9b2b20218538