Fix crash from DescendantInvalidationSet::show().

Adding a dictionary without a name to TracedValue without being inside an
array asserts. Added an array around the invalidation set to avoid that
crash from DescendantInvalidationSet::show() which is used for debugging.

R=kouhei@chromium.org
BUG=425577

Review URL: https://codereview.chromium.org/637383004

git-svn-id: svn://svn.chromium.org/blink/trunk@184097 bbb929c8-8fbe-4397-9dbb-9b2b20218538