ASSERT that SecurityError exceptions are thrown via ::throwSecurityError.

This will help us ensure that security-relevant exception messages (which
can be exposed to JavaScript) are sufficiently sanitized. Unsanitized
messages with additional detail may be exposed to the console.

BUG=152212

Review URL: https://codereview.chromium.org/24352003

git-svn-id: svn://svn.chromium.org/blink/trunk@159434 bbb929c8-8fbe-4397-9dbb-9b2b20218538