cc: Add image to rect map in DiscardableImageMap.

cc/playback/image_id.h

Index: cc/playback/image_id.h
diff --git a/cc/playback/image_id.h b/cc/playback/image_id.h
new file mode 100644
index 0000000000000000000000000000000000000000..c29810b6746dc961d545d0dca387793b00b8c951
--- /dev/null
+++ b/cc/playback/image_id.h
@@ -0,0 +1,14 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.

[vmpstr, 2017/02/01 01:16:45]

I don't know how I feel about a file just for a single typedef. 

Can we maybe tack this on in draw_image.h? (not as a part of DrawImage, but just
as a separate entry)

[Khushal, 2017/02/01 01:44:52]

Hmmm, it doesn't have anything to do with DrawImage. Also I figured it was okay
since we have other stuff like this:
https://cs.chromium.org/chromium/src/cc/base/resource_id.h. We'll eventually end
up replacing this with something in cc/paint anyway.

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CC_PLAYBACK_IMAGE_ID_H_
+#define CC_PLAYBACK_IMAGE_ID_H_
+
+namespace cc {
+
+using ImageId = uint32_t;
+
+}  // namespace cc
+
+#endif  // CC_PLAYBACK_IMAGE_ID_H_