When the security level is HTTP_SHOW_WARNING, we may add an explanation
to the Developer Tools console. We must ensure that we only add the
Form Not Secure explanation if the markup displayed a sensitive input
field.

Also remove adjacent dead code related to the completed field trial; Form Not Secure is always on now.

BUG=691412
TEST=SecurityStateTabHelperTest.*

TBR=sdefresne@chromium.org

Review-Url: https://codereview.chromium.org/2727353002
Cr-Commit-Position: refs/heads/master@{#454443}