Refactor the way PaintArtifactCompositor is attached and used.

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 048e7d1c315d85fd90c25e562ffe98c1cde3a5ad..8ed7a77c54df04256c3861f4568d6422eb237eee 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -106,6 +106,7 @@
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/GraphicsLayer.h"
 #include "platform/graphics/GraphicsLayerDebugInfo.h"
+#include "platform/graphics/compositing/PaintArtifactCompositor.h"
 #include "platform/graphics/paint/CullRect.h"
 #include "platform/graphics/paint/PaintController.h"
 #include "platform/graphics/paint/ScopedPaintChunkProperties.h"
@@ -2762,12 +2763,15 @@ void FrameView::pushPaintArtifactToCompositor()
 
     ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
 
-    const PaintArtifact& paintArtifact = m_paintController->paintArtifact();
+    if (!m_paintArtifactCompositor) {
+        m_paintArtifactCompositor = PaintArtifactCompositor::create();
+        Page* page = frame().page();
+        if (!page)

[pdr., 2016/09/15 20:53:07]

Can this ever happen? If it does, wouldn't we forget to attach the root layer?

[chrishtr, 2016/09/15 21:05:39]

I don't know if it can happen. What I can say is that there are several places
that defensively return on null page() already in this class, 
including jbroman's here (which was probably also cargo-culted).

Yes it means we won't attach, but that's ok because there is no page to attach
to anyway. :)

[pdr., 2016/09/15 21:08:17]

Why don't we just bail then, instead of creating and updating the paint artifact
compositor?

[chrishtr, 2016/09/15 21:11:04]

Done.

+            return;
 
-    Page* page = frame().page();
-    if (!page)
-        return;
-    page->chromeClient().didPaint(paintArtifact);
+        page->chromeClient().attachRootLayer(m_paintArtifactCompositor->getWebLayer(), &frame());
+    }
+    m_paintArtifactCompositor->update(m_paintController->paintArtifact());
 }
 
 void FrameView::updateStyleAndLayoutIfNeededRecursive()