Clear m_lastImageId for software compositor

Source/platform/graphics/Canvas2DLayerBridge.cpp

Index: Source/platform/graphics/Canvas2DLayerBridge.cpp
diff --git a/Source/platform/graphics/Canvas2DLayerBridge.cpp b/Source/platform/graphics/Canvas2DLayerBridge.cpp
index af5e9e4ab5bd3d6708490c711e9a6b633bbf0bb0..05b88daaf5d5cd147a57c2b9af14249608fa627c 100644
--- a/Source/platform/graphics/Canvas2DLayerBridge.cpp
+++ b/Source/platform/graphics/Canvas2DLayerBridge.cpp
@@ -343,10 +343,11 @@ bool Canvas2DLayerBridge::prepareMailbox(blink::WebExternalTextureMailbox* outMa
 {
     if (bitmap) {
         // Using accelerated 2d canvas with software renderer, which
-        // should only happen in tests that use fake graphics contexts.
-        // In this case, we do not care about producing any results for
-        // compositing.
+        // should only happen in tests that use fake graphics contexts
+        // or in Android WebView in software mode. In this case, we do
+        // not care about producing any results for this canvas.
         m_canvas->silentFlush();
+        m_lastImageId = 0;
         return false;
     }
     if (!isValid())