Drop mixed content callbacks from layout tests.

Mixed content callbacks are currently dumped as part of 'dumpFrameLoadCallbacks'
which is causing flake in layout tests whose load order is (for whatever reason)
indeterminate. 'security/mixedContent/data-url-iframe-in-main-frame.html' is a
good example.

Since the frame load callbacks are a) verbose, and b) not particularly relevant
to what the tests ought to be testing, we can avoid the flake by reducing the
scope of the tests' output. Mixed content errors always generate console
messages, which means that dropping the callbacks entirely is both possible
and advisable. This patch does so.

BUG=239811
R=abarth@chromium.org,jochen@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19547002

git-svn-id: svn://svn.chromium.org/blink/trunk@154484 bbb929c8-8fbe-4397-9dbb-9b2b20218538