PrepareMailbox when all mailboxes have been returned

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 8a3f810532ede8abeac0d74831a239ff53d77922..638cd4b77ae78bf055967a8cad973ed1bed9847d 100644
--- a/Source/platform/graphics/Canvas2DLayerBridge.cpp
+++ b/Source/platform/graphics/Canvas2DLayerBridge.cpp
@@ -93,6 +93,7 @@ Canvas2DLayerBridge::Canvas2DLayerBridge(PassOwnPtr<blink::WebGraphicsContext3DP
     , m_prev(0)
     , m_lastImageId(0)
     , m_releasedMailboxInfoIndex(InvalidMailboxIndex)
+    , m_activeMailboxCount(0)
 {
     ASSERT(m_canvas);
     ASSERT(m_contextProvider);
@@ -369,6 +370,7 @@ bool Canvas2DLayerBridge::prepareMailbox(blink::WebExternalTextureMailbox* outMa
     MailboxInfo* mailboxInfo = createMailboxInfo();
     mailboxInfo->m_status = MailboxInUse;
     mailboxInfo->m_image = image;
+    m_activeMailboxCount++;
 
     // Because of texture sharing with the compositor, we must invalidate
     // the state cached in skia so that the deferred copy on write
@@ -439,6 +441,9 @@ void Canvas2DLayerBridge::mailboxReleased(const blink::WebExternalTextureMailbox
             mailboxInfo->m_mailbox.syncPoint = mailbox.syncPoint;
             ASSERT(mailboxInfo->m_status == MailboxInUse);
             mailboxInfo->m_status = MailboxReleased;
+            ASSERT(m_activeMailboxCount > 0);
+            if (!--m_activeMailboxCount)
+                m_lastImageId = 0;
             // Trigger Canvas2DLayerBridge self-destruction if this is the
             // last live mailbox and the layer bridge is not externally
             // referenced.