Don't create render passes for transparent images.

cc/layer_tree_host_common.cc

Index: cc/layer_tree_host_common.cc
diff --git a/cc/layer_tree_host_common.cc b/cc/layer_tree_host_common.cc
index ca7d45050f85c0857e5f88a4def5818ef1b5374e..771a6ae47ab590372751bcd11e64b414cd4cd6ff 100644
--- a/cc/layer_tree_host_common.cc
+++ b/cc/layer_tree_host_common.cc
@@ -141,6 +141,26 @@ static gfx::Rect calculateVisibleContentRect(LayerType* layer)
     return LayerTreeHostCommon::calculateVisibleRect(targetSurfaceClipRect, gfx::Rect(gfx::Point(), layer->contentBounds()), layer->drawTransform());
 }
 
+// Note that this function is short circuiting for speed. When the number of
+// descendants that draw content is >1, the result if guaranteed to by >1 but
+// not the actual number of descendants that draw content.
+template<typename LayerType>
+static int getNumDescendantsDrawContent(LayerType* layer)
+{
+    typedef LayerType* LayerTypePtr;
+    const typename LayerType::LayerList& children = layer->children();
+
+    int result = 0;
+    for (size_t i = 0; i < children.size(); ++i) {
+        if (children[i]->drawsContent())
+            ++result;
+        result += getNumDescendantsDrawContent(LayerTypePtr(children[i]));
+        if (result > 1)
+            return result;
+    }
+    return result;
+}
+
 static bool isScaleOrTranslation(const WebTransformationMatrix& m)
 {
     return !m.m12() && !m.m13() && !m.m14()
@@ -247,7 +267,8 @@ static bool subtreeShouldRenderToSeparateSurface(LayerType* layer, bool axisAlig
         return true;
 
     // Cache this value, because otherwise it walks the entire subtree several times.
-    bool descendantDrawsContent = layer->descendantDrawsContent();
+    int numDescendantsDrawContent = getNumDescendantsDrawContent(layer);
+    bool descendantDrawsContent = numDescendantsDrawContent > 0;
 
     // If the layer flattens its subtree (i.e. the layer doesn't preserve-3d), but it is
     // treated as a 3D object by its parent (i.e. parent does preserve-3d).
@@ -259,7 +280,8 @@ static bool subtreeShouldRenderToSeparateSurface(LayerType* layer, bool axisAlig
         return true;
 
     // If the layer has opacity != 1 and does not have a preserves-3d transform style.
-    if (layer->opacity() != 1 && !layer->preserves3D() && descendantDrawsContent)
+    if (layer->opacity() != 1 && !layer->preserves3D() && descendantDrawsContent
+        && (layer->drawsContent() || numDescendantsDrawContent > 1))
         return true;
 
     return false;
@@ -484,10 +506,10 @@ static void calculateDrawTransformsInternal(LayerType* layer, const WebTransform
 
     gfx::Rect clipRectForSubtree;
     bool subtreeShouldBeClipped = false;
-    
+
     float drawOpacity = layer->opacity();
     bool drawOpacityIsAnimating = layer->opacityIsAnimating();
-    if (layer->parent() && layer->parent()->preserves3D()) {
+    if (layer->parent()) {
         drawOpacity *= layer->parent()->drawOpacity();
         drawOpacityIsAnimating |= layer->parent()->drawOpacityIsAnimating();
     }