Add some additional SMIL use counters

To gain a better understanding of the usage of particular features of SMIL
animations, this patch adds use counters for the following conditions:
- event-value used in 'begin' or 'end' property,
- syncbase-value used in 'begin' or 'end' property, and
- SMIL element inserted into document after load event has fired.

Review-Url: https://codereview.chromium.org/2132323002
Cr-Commit-Position: refs/heads/master@{#406800}
6 files changed