PropertyCallbackInfo::This can be a primitive value.

Update all callsites to explicitly check for Object first

BUG=v8:3274
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/246713004

git-svn-id: svn://svn.chromium.org/blink/trunk@172145 bbb929c8-8fbe-4397-9dbb-9b2b20218538