Optimize memory use in DescendantInvalidationSet.

Don't allocate a HashMap unless there is something to put in it.

BUG=335247

Review URL: https://codereview.chromium.org/173093002

git-svn-id: svn://svn.chromium.org/blink/trunk@167554 bbb929c8-8fbe-4397-9dbb-9b2b20218538