Refactor mixed content checks against the top frame into MixedContentChecker.

We shouldn't force the callsites to understand that they need to check the
top-level frame. It leads to bugs and oversights, at least one of which is
fixed by this patch.

BUG=390123

Review URL: https://codereview.chromium.org/348853009

git-svn-id: svn://svn.chromium.org/blink/trunk@177285 bbb929c8-8fbe-4397-9dbb-9b2b20218538