cc: Fix occlusion tracking for layers with non-default blend modes

When computing the occlusion from a layer, draw_blend_mode rather than
blend_mode needs to be considered, since draw_blend_mode is what's used
when drawing the layer. However, non-default blend modes currently always
trigger surface creation, and then the blend mode gets applied when
drawing the surface. This means that draw_blend_mode is always SrcOver.
This CL replaces the incorrect use of blend_mode in
OcclusionTracker::MarkOcclusionBehindLayer with a DCHECK that
draw_blend_mode is SrcOver.

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2176903002
Cr-Commit-Position: refs/heads/master@{#407572}

cc/trees/occlusion_tracker.cc

@@ -350,8 +350,9 @@ void OcclusionTracker::MarkOccludedBehindLayer(const LayerImpl* layer) {
   if (layer->draw_opacity() < 1)
     return;
 
-  if (!layer->uses_default_blend_mode())
-    return;
+  // The only currently supported draw_blend_mode is SrcOver mode, so
+  // draw_blend_mode does not affect occlusion.
+  DCHECK_EQ(layer->draw_blend_mode(), SkXfermode::kSrcOver_Mode);
 
   if (layer->Is3dSorted())
     return;

cc/trees/occlusion_tracker_unittest.cc

@@ -1986,10 +1986,13 @@ class OcclusionTrackerTestBlendModeDoesNotOcclude
     EXPECT_TRUE(occlusion.occlusion_from_outside_target().IsEmpty());
 
     this->VisitLayer(blend_mode_layer, &occlusion);
-    // |top_layer| occludes but not |blend_mode_layer|.
+    // |top_layer| and |blend_mode_layer| both occlude, since the blend mode
+    // gets applied by blend_mode_layer's render surface, not when drawing the
+    // layer itself.
+    EXPECT_EQ(gfx::Rect(100, 100).ToString(),
+              occlusion.occlusion_from_inside_target().ToString());
     EXPECT_EQ(gfx::Rect(10, 12, 20, 22).ToString(),
               occlusion.occlusion_from_outside_target().ToString());
-    EXPECT_TRUE(occlusion.occlusion_from_inside_target().IsEmpty());
 
     this->VisitContributingSurface(blend_mode_layer, &occlusion);
     // |top_layer| occludes but not |blend_mode_layer|.