Add support for Low Priorirty tasks.

Source/wtf/DoubleBufferedDequeTest.cpp

Index: Source/wtf/DoubleBufferedDequeTest.cpp
diff --git a/Source/wtf/DoubleBufferedDequeTest.cpp b/Source/wtf/DoubleBufferedDequeTest.cpp
index 5c56ae93567709023002eb02976efa5732825602..5f717d769ed9d65ad115139dfe1c345c9e643cfe 100644
--- a/Source/wtf/DoubleBufferedDequeTest.cpp
+++ b/Source/wtf/DoubleBufferedDequeTest.cpp
@@ -50,4 +50,21 @@ TEST(DoubleBufferedDequeTest, TestDoubleBuffering)
     EXPECT_EQ(3, queue.swapBuffers().takeFirst());
 }
 
+TEST(DoubleBufferedDequeTest, TestAllQueuesSize)
+{
+    DoubleBufferedDeque<int> queue;
+
+    EXPECT_EQ(0U, queue.allQueuesSize());
+
+    queue.append(10);
+    queue.append(20);
+    queue.append(30);
+
+    EXPECT_EQ(3U, queue.allQueuesSize());
+    queue.swapBuffers();
+
+    queue.append(10);
+    EXPECT_EQ(4U, queue.allQueuesSize());
+}
+
 } // namespace