Tests active mixed content blocking in the security panel.

<DIV class=security-explanation security-explanation-info >
    <DEVTOOLS-ICON role=presentation name=indeterminate-question-box class=security-property security-property-info >
        <#document-fragment >
            <STYLE >
            </STYLE>
            <SPAN style=--icon-url: var(--image-file-indeterminate-question-box); >
            </SPAN>
        </#document-fragment>
    </DEVTOOLS-ICON>
    <DIV class=security-explanation-text >
        <DIV class=security-explanation-title >
Blocked mixed content
        </DIV>
        <DIV >
Your page requested non-secure resources that were blocked.
        </DIV>
        <BUTTON class=security-mixed-content devtools-link text-button link-style role=link tabindex=0 >
View 1 request in Network panel
        </BUTTON>
    </DIV>
</DIV>