Port refcount-leaks to PPAPI

Trying to keep the spirit of the test, but refcount is not exposed to the plugin in PPAPI, so just focus on the instance count.

BUG=318978,493212,474535

Review URL: https://codereview.chromium.org/1840963002

Cr-Commit-Position: refs/heads/master@{#383677}
3 files changed