Clear m_lastImageId for software compositor

Source/web/tests/Canvas2DLayerBridgeTest.cpp

Index: Source/web/tests/Canvas2DLayerBridgeTest.cpp
diff --git a/Source/web/tests/Canvas2DLayerBridgeTest.cpp b/Source/web/tests/Canvas2DLayerBridgeTest.cpp
index 9d1b37cd55e68c0787697f6e0d1641e29e7eae1b..9f301080e8cab313d014281e9e773223d8f539a4 100644
--- a/Source/web/tests/Canvas2DLayerBridgeTest.cpp
+++ b/Source/web/tests/Canvas2DLayerBridgeTest.cpp
@@ -30,6 +30,7 @@
 #include "SkSurface.h"
 #include "platform/graphics/ImageBuffer.h"
 #include "public/platform/Platform.h"
+#include "public/platform/WebExternalBitmap.h"
 #include "public/platform/WebGraphicsContext3DProvider.h"
 #include "public/platform/WebThread.h"
 #include "third_party/skia/include/core/SkDevice.h"
@@ -90,6 +91,23 @@ private:
     RefPtr<Canvas2DLayerBridge> m_layerBridge;
 };
 
+class NullWebExternalBitmap : public WebExternalBitmap {
+public:
+    virtual WebSize size()
+    {
+        return WebSize();
+    }
+
+    virtual void setSize(WebSize)
+    {
+    }
+
+    virtual uint8* pixels()
+    {
+        return 0;
+    }
+};
+
 } // namespace
 
 class Canvas2DLayerBridgeTest : public Test {
@@ -116,6 +134,19 @@ protected:
 
         ::testing::Mock::VerifyAndClearExpectations(&mainMock);
     }
+
+    void prepareMailboxWithBitmapTest()
+    {
+        MockCanvasContext mainMock;
+        OwnPtr<SkDeferredCanvas> canvas = adoptPtr(SkDeferredCanvas::Create(SkSurface::NewRasterPMColor(300, 150)));
+        OwnPtr<MockWebGraphicsContext3DProvider> mainMockProvider = adoptPtr(new MockWebGraphicsContext3DProvider(&mainMock));
+        Canvas2DLayerBridgePtr bridge(adoptRef(new Canvas2DLayerBridge(mainMockProvider.release(), canvas.release(), 0, NonOpaque)));
+        bridge->m_lastImageId = 1;
+
+        NullWebExternalBitmap bitmap;
+        bridge->prepareMailbox(0, &bitmap);
+        EXPECT_EQ(0u, bridge->m_lastImageId);
+    }
 };
 
 namespace {
@@ -125,4 +156,9 @@ TEST_F(Canvas2DLayerBridgeTest, testFullLifecycleSingleThreaded)
     fullLifecycleTest();
 }
 
+TEST_F(Canvas2DLayerBridgeTest, prepareMailboxWithBitmapTest)
+{
+    prepareMailboxWithBitmapTest();
+}
+
 } // namespace