Some tests passed a string as second argument to assertThrows, expecting it to
be matched against the exception.  However, assertThrows simply ignored these.
(Some other tests actually seem to use that argument as a comment ...)

This CL
- changes assertThrows to fail if the second argument is not a function,
- adds assertThrowsEquals which compares the exception to a given value using
  assertEquals
- fixes some bogus tests that got exposed by this.

R=jarin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1544793002

Cr-Commit-Position: refs/heads/master@{#33159}