Enable fastUseReject for tree boundary crossing rules.

Tree boundary crossing rules have one of ::content, ^, or ^^.
We can enable the filter for rules containing ^ or ^^,
but cannot enable the filter for rules containing ::content.

To disable the filter for ::content, just destroy hash table in collectIdentifierHashes
when finding ::content.

BUG=314300

Review URL: https://codereview.chromium.org/74783005

git-svn-id: svn://svn.chromium.org/blink/trunk@162288 bbb929c8-8fbe-4397-9dbb-9b2b20218538