Add link preload as=document support

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

Index: third_party/WebKit/LayoutTests/http/tests/preload/onload_event.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/preload/onload_event.html b/third_party/WebKit/LayoutTests/http/tests/preload/onload_event.html
index 7ff00bc420bebb156ff02e404f5b2e09019c6731..6a6028aa8f282d8e24290ff2a1cdaf1f955e86cf 100644
--- a/third_party/WebKit/LayoutTests/http/tests/preload/onload_event.html
+++ b/third_party/WebKit/LayoutTests/http/tests/preload/onload_event.html
@@ -13,6 +13,7 @@
     var videoLoaded = false;
     var audioLoaded = false;
     var trackLoaded = false;
+    var iframeLoaded = false;
     var gibberishLoaded = false;
     var gibberishErrored = false;
     var noTypeLoaded = false;
@@ -24,6 +25,7 @@
 <link rel=preload href="../resources/test.mp4" as=media onload="videoLoaded = true;">
 <link rel=preload href="../resources/test.oga" as=media onload="audioLoaded = true;">
 <link rel=preload href="../security/resources/captions.vtt" as=track onload="trackLoaded = true;">
+<link rel=preload href="../resources/test.html" onload="iframeLoaded = true;">
 <link rel=preload href="../resources/dummy.xml" as=foobarxmlthing onload="gibberishLoaded = true;" onerror="gibberishErrored = true;">
 <link rel=preload href="../resources/dummy.xml" onload="noTypeLoaded = true;">
 <script src="../resources/slow-script.pl?delay=500"></script>
@@ -36,6 +38,7 @@
         assert_true(videoLoaded, "video triggered load event");
         assert_true(audioLoaded, "audio triggered load event");
         assert_true(trackLoaded, "track triggered load event");
+        assert_true(iframeLoaded, "iframe triggered load event");
         assert_false(gibberishLoaded, "gibberish as value triggered load event");
         assert_true(gibberishErrored, "gibberish as value triggered error event");
         assert_true(noTypeLoaded, "empty as triggered load event");