Implements CSSPropertyAPI for the offset-position property.

third_party/WebKit/Source/core/css/properties/CSSPropertyAPIOffsetPosition.cpp

Index: third_party/WebKit/Source/core/css/properties/CSSPropertyAPIOffsetPosition.cpp
diff --git a/third_party/WebKit/Source/core/css/properties/CSSPropertyAPIOffsetPosition.cpp b/third_party/WebKit/Source/core/css/properties/CSSPropertyAPIOffsetPosition.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..74f2eb7aba753c5f9e5a477e42c9a9416c4010f2
--- /dev/null
+++ b/third_party/WebKit/Source/core/css/properties/CSSPropertyAPIOffsetPosition.cpp
@@ -0,0 +1,29 @@
+// Copyright 2017 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 "core/css/properties/CSSPropertyAPIOffsetPosition.h"
+
+#include "core/css/CSSValuePair.h"
+#include "core/css/parser/CSSParserContext.h"
+#include "core/css/parser/CSSPropertyParserHelpers.h"
+#include "core/frame/UseCounter.h"
+
+namespace blink {
+
+const CSSValue* CSSPropertyAPIOffsetPosition::parseSingleValue(
+    CSSParserTokenRange& range,
+    const CSSParserContext& context) {
+  CSSValueID id = range.peek().id();
+  if (id == CSSValueAuto)
+    return CSSPropertyParserHelpers::consumeIdent(range);
+  CSSValue* value = CSSPropertyParserHelpers::consumePosition(
+      range, context.mode(), CSSPropertyParserHelpers::UnitlessQuirk::Forbid);
+
+  // Count when we receive a valid position other than 'auto'.
+  if (context.useCounter() && value && value->isValuePair())
+    context.useCounter()->count(UseCounter::CSSOffsetInEffect);
+  return value;
+}
+
+}  // namespace blink