Use element id's for animations

third_party/WebKit/Source/platform/graphics/CompositorElementId.cpp

Index: third_party/WebKit/Source/platform/graphics/CompositorElementId.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/CompositorElementId.cpp b/third_party/WebKit/Source/platform/graphics/CompositorElementId.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..9b2c249db6f61176633b549b6921d987a9abc24d
--- /dev/null
+++ b/third_party/WebKit/Source/platform/graphics/CompositorElementId.cpp
@@ -0,0 +1,16 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "platform/graphics/CompositorElementId.h"
+
+namespace blink {
+
+CompositorElementId createCompositorElementId(int domNodeId, CompositorSubElementId::Value subElementId)
+{
+    uint64_t value = domNodeId;

[jbroman, 2016/06/02 20:20:21]

I guess we're assuming here that domNodeId < (1 << 32UL)? Otherwise the failure
mode here seems pretty confusing.

If you can assume this statically (i.e. if we have no platforms where
sizeof(int) > sizeof(int32_t)), a static_assert here would be great in
explaining on what basis we assume this to be safe, i.e.

static_assert(sizeof(domNodeId) <= 4, "DOM node IDs are assumed not to be larger
than 32 bits");

If that's not the case, a comment and/or DCHECK would be appreciated.

[Ian Vollick, 2016/06/03 19:31:21]

This is indeed dodgy. I've changed the innards of ElementIds to be two ints to
avoid problems like this.

+    value |= static_cast<uint64_t>(subElementId) << 32UL;
+    return CompositorElementId(value);
+}
+
+} // namespace blink