Fix the context loss issue when switching tabs after enabled Canvas2D hardware acceleration

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..eb20433db055f675ca7e658e3a7fe58c9f1d212f 100644
--- a/Source/platform/graphics/Canvas2DLayerBridge.cpp
+++ b/Source/platform/graphics/Canvas2DLayerBridge.cpp
@@ -273,6 +273,7 @@ bool Canvas2DLayerBridge::hasReleasedMailbox() const
 void Canvas2DLayerBridge::freeReleasedMailbox()
 {
     MailboxInfo* mailboxInfo = releasedMailboxInfo();
+    m_lastImageId = -1;

[boliu, 2014/02/11 19:29:41]

We only want to do this if the released mailbox matches m_lastImageId. You can
do this by comparing m_lastImageId to mailboxInfo->SkImage->uniqueID(),
obviously with all the proper null checks in place.

Also released here means calling Canvas2DLayerBridge::mailboxReleased (ie
release back to us by compositor), not here, actually freeing the mailbox. So
move this to Canvas2DLayerBridge::mailboxReleased.

     if (!mailboxInfo)
         return;