Slimming Paint phase 2 compositing algorithm plumbing & skeleton display list API.

Source/platform/graphics/paint/DisplayItemList.h

Index: Source/platform/graphics/paint/DisplayItemList.h
diff --git a/Source/platform/graphics/paint/DisplayItemList.h b/Source/platform/graphics/paint/DisplayItemList.h
index 133e2f8e0b18840e0b0b2a9cdfefe8b825098e20..a1f98e272921c3b18df4621b32fbf41f3c464a05 100644
--- a/Source/platform/graphics/paint/DisplayItemList.h
+++ b/Source/platform/graphics/paint/DisplayItemList.h
@@ -23,6 +23,10 @@ using DisplayItems = ListContainer<DisplayItem>;
 static const size_t kInitialDisplayItemsCapacity = 64;
 static const size_t kMaximumDisplayItemSize = sizeof(BeginTransform3DDisplayItem);
 
+class DisplayListDiff {
+    // TODO(chrishtr): define and implement the diff structure.
+};
+
 class PLATFORM_EXPORT DisplayItemList {
     WTF_MAKE_NONCOPYABLE(DisplayItemList);
     WTF_MAKE_FAST_ALLOCATED(DisplayItemList);
@@ -60,8 +64,9 @@ public:
     void endSkippingCache() { ASSERT(m_skippingCacheCount > 0); --m_skippingCacheCount; }
     bool skippingCache() const { return m_skippingCacheCount; }
 
-    // Must be called when a painting is finished.
-    void commitNewDisplayItems();
+    // Must be called when a painting is finished. If passed, the diff is initialized
+    // to the difference of the new display list from the last time commitNewDisplayItems() was called.
+    void commitNewDisplayItems(DisplayListDiff* = 0);

[weiliangc, 2015/07/24 19:17:15]

What owns the DisplayListDiff? Root Graphics Layer?

[chrishtr, 2015/07/24 19:27:22]

The caller of commitNewDisplayItems owns it locally. In this CL the diff is not
part of the API to cc, the API is still:

1. a vector of cc::Layer*'s, on which one stores the usual bits, including
crucially raster invalidation rects. These rects are derived from the
DisplayListDiff as interpreted by the DisplayListCompositingBuilder.

2. property trees (stored on the LTH probably)

 
     // Returns the approximate memory usage, excluding memory likely to be
     // shared with the embedder after copying to WebDisplayItemList.