Added CSSPropertyAPI and CSS padding properties which implement this API

third_party/WebKit/Source/core/css/properties/CSSPropertyDescriptor.h

Index: third_party/WebKit/Source/core/css/properties/CSSPropertyDescriptor.h
diff --git a/third_party/WebKit/Source/core/css/properties/CSSPropertyDescriptor.h b/third_party/WebKit/Source/core/css/properties/CSSPropertyDescriptor.h
new file mode 100644
index 0000000000000000000000000000000000000000..473270ec413d92d81537d3fad235f9d058782c66
--- /dev/null
+++ b/third_party/WebKit/Source/core/css/properties/CSSPropertyDescriptor.h
@@ -0,0 +1,31 @@
+// 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 "core/CSSPropertyNames.h"
+
+namespace blink {
+
+class CSSValue;
+class CSSParserTokenRange;
+class CSSParserContext;
+
+// Stores function pointers matching those declared in CSSPropertyAPI.
+struct CSSPropertyDescriptor {
+  const CSSValue* (*parseSingleValue)(CSSParserTokenRange&,
+                                      const CSSParserContext&);
+  CSSPropertyID id;
+
+  // Stores whether or not this descriptor is for a valid property. Do not
+  // access the contents of this descriptor unless this value is true.
+  // TODO(aazzam): Remove this once the API has been implemented for all
+  // properties.
+  bool isValid;

[Z_DONOTUSE, 2016/12/08 01:00:18]

Same here. If this is temporary, let's make it clear in the name so that other
code is less likely to depend on it. Also, it can be less general.

Something like, temporaryCanReadValue. I'm intentionally going with something
that's a little confusing so that people won't be tempted to use it for
anything. :)

[aazzam, 2016/12/08 02:27:10]

On second thoughts this isn't that temporary - eventually we will implement most
properties, however we still will may want to not implement some properties and
will need this variable to check. I've removed the TODO which was there before
:)

[Z_DONOTUSE, 2016/12/08 03:18:00]

What are cases where we'd want to keep this? Sorry for all the questions. We've
just had a history of overly generic names getting misused. I don't know what
makes a descriptor valid or invalid, so other people probably won't either. I'm
trying to understand the end design we're aiming for.

Also, there's an extra branch for the isValid check that we don't necessarily
need.

[aazzam, 2016/12/08 04:16:36]

Okay - for now, since we have properties which we intend to still implement, we
need to keep isValid as a temporary variable. I've renamed it (as suggested) to
temporaryCanReadValue :)

In the future (once we've implemented all properties), I will make a new struct
CSSInvalidPropertyDescriptor which implements the methods with NOTREACHED() for
invalid properties where we want a runtime error to occur. 
For properties which aren't invalid, but are not handled in the switch statement
(the ones which go to the default: return nullptr;) I will implement them with
implementation return nullptr.
So our final design of CSSPropertyParser won't have the branching for isValid
check, and will also have the entire switch statement removed.

+
+  // Returns the corresponding CSSPropertyDescriptor for a given CSSPropertyID.
+  // Use this function to access the API for a property. Returns a descriptor
+  // with isValid set to false if no descriptor exists for this ID.
+  static const CSSPropertyDescriptor& get(CSSPropertyID);
+};
+
+}  // namespace blink