Don't cache subsequence whose layer is not fully contained by repaint rect

Source/platform/graphics/paint/DisplayItemList.cpp

Index: Source/platform/graphics/paint/DisplayItemList.cpp
diff --git a/Source/platform/graphics/paint/DisplayItemList.cpp b/Source/platform/graphics/paint/DisplayItemList.cpp
index 6f2a8753ee589bad8d912050f607ba5bd7a32cd3..90d34ce62bc17ce773fd6eb54167f498e532eec3 100644
--- a/Source/platform/graphics/paint/DisplayItemList.cpp
+++ b/Source/platform/graphics/paint/DisplayItemList.cpp
@@ -94,8 +94,11 @@ void DisplayItemList::processNewItem(DisplayItem& displayItem)
 #endif // ENABLE(ASSERT)
 
     ASSERT(!displayItem.skippedCache()); // Only DisplayItemList can set the flag.
-    if (skippingCache())
+    if (skippingCache()) {
+        // TODO(wangxianzhu): Distinguish skippingCache and uncacheablility.

[chrishtr, 2015/09/08 22:48:00]

What's the difference exactly?

[Xianzhu, 2015/09/09 16:50:33]

Wanted to distinguish "skip-cache but cacheable" and "uncachable". However, the
former is not used for now so I removed uncacheable flag in the new patch. (I
planned to use the former to optimize full-subtree invalidation to avoid mark
every object for invalidation.)

         displayItem.setSkippedCache();
+        displayItem.setUncacheable();
+    }
 }
 
 void DisplayItemList::beginScope()