Only track invalidations when causes are being tracked

This patch fixes a simple bug where invalidations were tracked when
causes were not tracked. This added unnecessary overhead when the
invalidation experiment was enabled but causes were not tracked.

TEST=manual
BUG=410701

Review URL: https://codereview.chromium.org/688853003

git-svn-id: svn://svn.chromium.org/blink/trunk@184745 bbb929c8-8fbe-4397-9dbb-9b2b20218538