Mixed Content: Make MixedContentChecker completely static.

This patch completes the transition away from MixedContentChecker as an
object with state, into a world where it just provides static methods.
This should clear up the interface, and make migration to RemoteFrame
a bit simpler.

After this patch, we no longer hang a MixedContentChecker object off
of FrameLoader, nor does MixedContentChecker need to hold a reference
to a frame. Everything we need to do the checks can be done by passing
values into the checker methods.

Note also that this patch deletes a number of expectation files: these
are testharness.js tests, which shouldn't have expectations
committed into the repository. Rather than modifying them, this patch
removes them.

As a drive-by, this also corrects the FormsSubmitted counter.
It ought to count all form submissions as "FormsSubmitted", and mixed
submissions as a subset thereof (e.g. 5 form submissions, 
with 2 of them mixed would result in FormsSubmitted: 5, 
MixedContentFormsSubmitted: 2, as opposed to the current 3 and 2).

BUG=411184

Review URL: https://codereview.chromium.org/566533003

git-svn-id: svn://svn.chromium.org/blink/trunk@181885 bbb929c8-8fbe-4397-9dbb-9b2b20218538