Block mixed <picture> and <img srcset> by default.

LayoutTests/http/tests/security/mixedContent/insecure-picture-in-main-frame-blocked.html

Index: LayoutTests/http/tests/security/mixedContent/insecure-picture-in-main-frame-blocked.html
diff --git a/LayoutTests/http/tests/security/mixedContent/insecure-css-in-main-frame.html b/LayoutTests/http/tests/security/mixedContent/insecure-picture-in-main-frame-blocked.html
similarity index 65%
copy from LayoutTests/http/tests/security/mixedContent/insecure-css-in-main-frame.html
copy to LayoutTests/http/tests/security/mixedContent/insecure-picture-in-main-frame-blocked.html
index f226ffbe1b2f0fdb02138fdb0d9792bf78af6e30..10425ada5e82aa1b7d9f95e681e28b281bd411a5 100644
--- a/LayoutTests/http/tests/security/mixedContent/insecure-css-in-main-frame.html
+++ b/LayoutTests/http/tests/security/mixedContent/insecure-picture-in-main-frame-blocked.html
@@ -6,6 +6,7 @@ if (window.testRunner) {
     testRunner.dumpAsText();
     testRunner.setCanOpenWindows();
     testRunner.setCloseRemainingWindowsWhenComplete(true);
+    testRunner.overridePreference("WebKitAllowDisplayingInsecureContent", false);
 }
 
 window.addEventListener("message", function (e) {
@@ -14,11 +15,9 @@ window.addEventListener("message", function (e) {
 }, false);
 
 </script>
-<p>This test opens a window that loads an insecure style sheet.  We should
-trigger a mixed content callback because the main frame in the window is HTTPS
-but is running insecure content.</p>
+<p>This test opens a window that loads an insecure picture.</p>
 <script>
-window.open("https://127.0.0.1:8443/security/mixedContent/resources/frame-with-insecure-css.html");
+window.open("https://127.0.0.1:8443/security/mixedContent/resources/frame-with-insecure-picture.html");
 </script>
 </body>
 </html>