Make -webkit-app-region CSS implementation self-consistent

The property -webkit-app-region went through renames and other changes.
It seems we can settle on none | drag | no-drag. The value none is used
internally as default, make it explicit by allowing it to be specified
in the style.

Bug: 803699

Change-Id: I1ec96c9e436c7595ac7ab34d90bfbfdd9ea48672
Reviewed-on: https://chromium-review.googlesource.com/964381Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Commit-Queue: Rob Buis <rob.buis@samsung.com>
Cr-Commit-Position: refs/heads/master@{#543671}

third_party/WebKit/LayoutTests/fast/css/getComputedStyle/computed-style-listing-expected.txt

@@ -3,7 +3,7 @@ This test documents all computed styles on a div element.
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
--webkit-app-region: no-drag
+-webkit-app-region: none
 -webkit-appearance: none
 -webkit-border-horizontal-spacing: 0px
 -webkit-border-image: none

third_party/WebKit/LayoutTests/fast/css/getComputedStyle/computed-style-without-renderer-listing-expected.txt

@@ -3,7 +3,7 @@ This test documents all computed styles on a display: none element.
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
--webkit-app-region: no-drag
+-webkit-app-region: none
 -webkit-appearance: none
 -webkit-border-horizontal-spacing: 0px
 -webkit-border-image: none

third_party/WebKit/LayoutTests/svg/css/getComputedStyle-listing-expected.txt

@@ -3,7 +3,7 @@ This test documents all computed styles on an SVG rect element.
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
--webkit-app-region: no-drag
+-webkit-app-region: none
 -webkit-appearance: none
 -webkit-border-horizontal-spacing: 0px
 -webkit-border-image: none

third_party/WebKit/Source/core/css/parser/CSSParserFastPaths.cpp

@@ -751,7 +751,8 @@ bool CSSParserFastPaths::IsValidKeywordPropertyAndValue(
       return value_id == CSSValueVisible || value_id == CSSValueHidden ||
              value_id == CSSValueCollapse;
     case CSSPropertyWebkitAppRegion:
-      return value_id >= CSSValueDrag && value_id <= CSSValueNoDrag;
+      return (value_id >= CSSValueDrag && value_id <= CSSValueNoDrag) ||
+             value_id == CSSValueNone;
     case CSSPropertyWebkitAppearance:
       return (value_id >= CSSValueCheckbox && value_id <= CSSValueTextarea) ||
              value_id == CSSValueNone;

third_party/WebKit/Source/core/css/properties/longhands/WebkitAppRegionCustom.cpp

@@ -16,6 +16,8 @@ const CSSValue* WebkitAppRegion::CSSValueFromComputedStyleInternal(
     const LayoutObject*,
     Node*,
     bool allow_visited_style) const {
+  if (style.DraggableRegionMode() == EDraggableRegionMode::kNone)
+    return CSSIdentifierValue::Create(CSSValueNone);
   return CSSIdentifierValue::Create(style.DraggableRegionMode() ==
                                             EDraggableRegionMode::kDrag
                                         ? CSSValueDrag