We have compatibility workarounds to return 'undefined' on accessors
to RegExp.prototype. This patch adds two UseCounters for two categories
of this non-spec-compliant path:
- source
- ignorecase, multiline, global

R=yangguo
BUG=chromium:581577
LOG=Y

Review URL: https://codereview.chromium.org/1762423002

Cr-Commit-Position: refs/heads/master@{#34597}