Add link preload as=document support

third_party/WebKit/LayoutTests/http/tests/preload/onerror_event.html

Index: third_party/WebKit/LayoutTests/http/tests/preload/onerror_event.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/preload/onerror_event.html b/third_party/WebKit/LayoutTests/http/tests/preload/onerror_event.html
index 802e2d6a38412cef0af81bfa69ae230dbb750ee0..f146b15cae905c3880eed2e03b19239ab84a64a9 100644
--- a/third_party/WebKit/LayoutTests/http/tests/preload/onerror_event.html
+++ b/third_party/WebKit/LayoutTests/http/tests/preload/onerror_event.html
@@ -13,6 +13,7 @@
     var videoFailed = false;
     var audioFailed = false;
     var trackFailed = false;
+    var iframeFailed = false;
     var gibrishFailed = false;
     var noTypeFailed = false;
 </script>
@@ -23,6 +24,7 @@
 <link rel=preload href="../non-existent/test.mp4" as=video onerror="videoFailed = true;">
 <link rel=preload href="../non-existent/test.oga" as=audio onerror="audioFailed = true;">
 <link rel=preload href="../non-existent/security/captions.vtt" as=track onerror="trackFailed = true;">
+<link rel=preload href="../non-existent/test.html" as=document onerror="iframeFailed = true;">
 <link rel=preload href="../non-existent/dummy.xml" as=foobarxmlthing onerror="gibrishFailed = true;">
 <link rel=preload href="../non-existent/dummy.xml" onerror="noTypeFailed = true;">
 <script src="../resources/slow-script.pl?delay=500"></script>
@@ -35,6 +37,7 @@
         assert_true(videoFailed, "video triggered error event");
         assert_true(audioFailed, "audio triggered error event");
         assert_true(trackFailed, "track triggered error event");
+        assert_true(iframeFailed, "iframe triggered error event");
         assert_true(gibrishFailed, "gibrish as value triggered error event");
         assert_true(noTypeFailed, "empty as triggered error event");
         t.done();