Free the command buffer when tabs are switched

gpu/command_buffer/client/cmd_buffer_helper.h

Index: gpu/command_buffer/client/cmd_buffer_helper.h
diff --git a/gpu/command_buffer/client/cmd_buffer_helper.h b/gpu/command_buffer/client/cmd_buffer_helper.h
index 5fbf4629ef6ec8261a5bcb24ef77864b0345c5ce..d27b25f2fc7c322755de1bded12868425a964a2e 100644
--- a/gpu/command_buffer/client/cmd_buffer_helper.h
+++ b/gpu/command_buffer/client/cmd_buffer_helper.h
@@ -218,6 +218,12 @@ class CommandBufferHelper {
     return ring_buffer_;
   }
 
+  void FreeRingBuffer();
+
+  bool HaveRingBuffer() const {
+    return ring_buffer_id_ != -1;
+  }
+
  private:
   // Waits until get changes, updating the value of get_.
   void WaitForGetChange();