Use StrictTypeChecking on EME objects.

With StrictTypeChecking, invalid types, null, etc. cannot reach the C++ code.

BUG=224791

Review URL: https://codereview.chromium.org/239083003

git-svn-id: svn://svn.chromium.org/blink/trunk@171878 bbb929c8-8fbe-4397-9dbb-9b2b20218538