Add cc::SharedBitmap::shared_memory_handle()

cc/resources/shared_bitmap.h

Index: cc/resources/shared_bitmap.h
diff --git a/cc/resources/shared_bitmap.h b/cc/resources/shared_bitmap.h
index 464ac5c6d4c020ed3cab876bc69c50a303843171..ac91c358c06694bf3f97e108065dfbd70ff10e43 100644
--- a/cc/resources/shared_bitmap.h
+++ b/cc/resources/shared_bitmap.h
@@ -14,6 +14,10 @@
 #include "gpu/command_buffer/common/mailbox.h"
 #include "ui/gfx/geometry/size.h"
 
+namespace base {
+class SharedMemoryHandle;
+}
+
 namespace cc {
 typedef gpu::Mailbox SharedBitmapId;
 
@@ -30,6 +34,10 @@ class CC_EXPORT SharedBitmap {
 
   const SharedBitmapId& id() { return id_; }
 
+  // Returns the shared memory's handle when the back end is base::SharedMemory.
+  // Otherwise, this returns an invalid handle.
+  virtual base::SharedMemoryHandle GetSharedMemoryHandle() const = 0;
+
   // Returns true if the size is valid and false otherwise.
   static bool SizeInBytes(const gfx::Size& size, size_t* size_in_bytes);
   // Dies with a CRASH() if the size can not be represented as a positive number