Pipeline caching: refactor object hashing

Dawn's object-based cache creates keys from memory
addresses. These keys cannot be used in a persistent
cache. This change modifies the keys to only
use hashes so they can be re-used for caching
pipelines.

BUG=dawn:549

Change-Id: Ica64d58ae6a3c6266435cfc3f776c820190f7895
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/30740
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>

src/dawn_native/ObjectContentHasher.cpp

@@ -0,0 +1,22 @@
+// Copyright 2020 The Dawn Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "dawn_native/ObjectContentHasher.h"
+
+namespace dawn_native {
+
+    size_t ObjectContentHasher::GetContentHash() const {
+        return mContentHash;
+    }
+}  // namespace dawn_native