[ObjC ARC] Converts ios/chrome/browser/passwords:unit_tests to ARC.

Notable issues:
 * After replacing scoped_nsobjects with strong ivars, the tests started failing
because of the change in destruction order of TestWebState and its observer.
Patched TestWebState to reset its observers upon destruction, similar to
WebStateImpl.

BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2559693003
Cr-Commit-Position: refs/heads/master@{#437223}
10 files changed