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..b39d5b17fe3ca6f3f4b6f378e3239c0f7e006719 100644
--- a/Source/platform/graphics/Canvas2DLayerBridge.cpp
+++ b/Source/platform/graphics/Canvas2DLayerBridge.cpp
@@ -439,6 +439,7 @@ void Canvas2DLayerBridge::mailboxReleased(const blink::WebExternalTextureMailbox
             mailboxInfo->m_mailbox.syncPoint = mailbox.syncPoint;
             ASSERT(mailboxInfo->m_status == MailboxInUse);
             mailboxInfo->m_status = MailboxReleased;
+            resetLastImageIdIfNecessary();
             // Trigger Canvas2DLayerBridge self-destruction if this is the
             // last live mailbox and the layer bridge is not externally
             // referenced.
@@ -457,6 +458,16 @@ void Canvas2DLayerBridge::mailboxReleased(const blink::WebExternalTextureMailbox
     }
 }
 
+void Canvas2DLayerBridge::resetLastImageIdIfNecessary()
+{
+    Vector<MailboxInfo>::iterator mailboxInfo;
+    for (mailboxInfo = m_mailboxes.begin(); mailboxInfo < m_mailboxes.end(); ++mailboxInfo) {

[Justin Novosad, 2014/02/13 15:32:18]

You can do the check in O(1) time instead of O(N) if you add a new data member
m_activeMailboxCount. You initialize it to zero, increment it in prepareMailbox,
decrement in mailboxReleased.

+        if (mailboxInfo->m_status == MailboxInUse)
+            return;
+    }
+    m_lastImageId = 0;
+}
+
 blink::WebLayer* Canvas2DLayerBridge::layer() const
 {
     ASSERT(m_layer);