Add casting helper functions for ImmutableStylePropertySet and MutableStylePropertySet.

They are helpful to detect bad casts.

BUG=309516

Review URL: https://codereview.chromium.org/63573007

git-svn-id: svn://svn.chromium.org/blink/trunk@162188 bbb929c8-8fbe-4397-9dbb-9b2b20218538