Fix progressive rendering of image documents

Source/web/tests/DocumentLoadingRenderingTest.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 
 #include "core/dom/Document.h"
 #include "core/frame/FrameView.h"
 #include "web/WebLocalFrameImpl.h"
 #include "web/tests/FrameTestHelpers.h"
@@ skipping 158 matching lines @@
 
     // Sheet finished, but no body since it's svg so don't resume.
     cssResource.finish();
     EXPECT_TRUE(m_layerTreeView.deferCommits());
 
     // Finish the load and resume.
     mainResource.finish();
     EXPECT_FALSE(m_layerTreeView.deferCommits());
 }
 
+TEST_F(DocumentLoadingRenderingTest, ShouldResumeImmediatelyForImageDocuments)
+{
+    SimRequest mainResource("https://example.com/test.png", "image/png");
+
+    loadURL("https://example.com/test.png");
+
+    mainResource.start();
+    EXPECT_TRUE(m_layerTreeView.deferCommits());
+
+    // Not really a valid image but enough for the test. ImageDocuments should
+    // resume painting as soon as the first bytes arrive.
+    mainResource.write("image data");
+    EXPECT_FALSE(m_layerTreeView.deferCommits());
+
+    mainResource.finish();
+    EXPECT_FALSE(m_layerTreeView.deferCommits());
+}
+
 TEST_F(DocumentLoadingRenderingTest, ShouldScheduleFrameAfterSheetsLoaded)
 {
     SimRequest mainResource("https://example.com/test.html", "text/html");
     SimRequest firstCssResource("https://example.com/first.css", "text/css");
     SimRequest secondCssResource("https://example.com/second.css", "text/css");
 
     loadURL("https://example.com/test.html");
 
     mainResource.start();
 
@@ skipping 88 matching lines @@
     cssResource.start();
     cssResource.finish();
 
     // First frame where all frames are loaded, should paint the text in the
     // child frame.
     auto frame3 = m_compositor.beginFrame();
     EXPECT_TRUE(frame3.containsText());
 }
 
 } // namespace blink