Fix crash in blink::Internals::numberOfScrollableAreas

The frame can be gone since there could have been a navigation that
removes it, as the testcase demonstrates. So like other methods
in Internals test for it first.

BUG=381066

Review URL: https://codereview.chromium.org/436673003

git-svn-id: svn://svn.chromium.org/blink/trunk@179367 bbb929c8-8fbe-4397-9dbb-9b2b20218538