Move V8ValueCache to new-style weak callbacks

BUG=324225
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/98883002

git-svn-id: svn://svn.chromium.org/blink/trunk@162982 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Get rid of extra NULL check when indexing CSSStyleDeclaration

In order to minimize the performance impact of added UseCounters in
https://codereview.chromium.org/47923015, remove one superfluous NULL
check. We always NULL check propInfo before use anyway.

BUG=313864

Review URL: https://codereview.chromium.org/98793002

git-svn-id: svn://svn.chromium.org/blink/trunk@162983 bbb929c8-8fbe-4397-9dbb-9b2b20218538