Don't count UseCounters in private scripts

When I shipped marquee-in-JS, it increased UseCounters for Web Animation APIs because the marquee-in-JS uses the Web Animation APIs. This was an unexpected outcome because what we want to count is the number of JS APIs used by web developers, not the number of JS APIs used internally (i.e., private scripts).

Given the above, this CL adds UseCounter::countIfNotPrivateScript() and uses it in a binding layer so that we can count only the number of JS APIs used by web developers.

BUG=341031

Review URL: https://codereview.chromium.org/656693002

git-svn-id: svn://svn.chromium.org/blink/trunk@183805 bbb929c8-8fbe-4397-9dbb-9b2b20218538