Revert of Implementing mixed content for forms posting to insecure location from secure ones (https://codereview.chromium.org/311033003/)

Reason for revert:
This CL incorrectly integrates with the inspector.

Notice that in Element::didModifyAttribute, InspectorInstrumentation::didModifyDOMAttr will be called with the original value, not the mutated value.

Original issue's description:
> Implementing mixed content for forms posting to insecure location from secure ones.
> 
> When a form "action" attribute is pointing to insecure location from a secure one, this is flagged as mixed content. This is even checked if the pages dynamically changes the "action" attribute after the initial loading. The mixed content is non-blocking by default, however, this can be overridden in the preferences, and this will cause a submit to silently fail, other than a console message.
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175714

TBR=jww@chromium.org,mkwst@chromium.org,mohammed@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/319183008

git-svn-id: svn://svn.chromium.org/blink/trunk@175715 bbb929c8-8fbe-4397-9dbb-9b2b20218538