Treat zero-width collapsed borders as invisible

We have fast-path for invisible collapsed borders. Treating zero-
width as invisible can improve performance in the case.

Also move core/style/CollapsedBorderValue.h to core/layout because
the structure is not used by any style code.

This CL will reduce frame time of PerformanceTests/Mutation/background-change-with-zero-width-collapsed-borders.html (in
https://codereview.chromium.org/2842313002/) by about 50%.

Review-Url: https://codereview.chromium.org/2842323002
Cr-Commit-Position: refs/heads/master@{#467741}

third_party/WebKit/Source/core/layout/BUILD.gn

@@ -14,6 +14,7 @@ blink_core_sources("layout") {
     "BidiRun.h",
     "BidiRunForLine.cpp",
     "BidiRunForLine.h",
+    "CollapsedBorderValue.h",
     "ColumnBalancer.cpp",
     "ColumnBalancer.h",
     "ContentChangeType.h",

third_party/WebKit/Source/core/style/CollapsedBorderValue.h → third_party/WebKit/Source/core/layout/CollapsedBorderValue.h

@@ -30,10 +30,21 @@
 
 namespace blink {
 
+enum EBorderPrecedence {
+  kBorderPrecedenceOff,
+  kBorderPrecedenceTable,
+  kBorderPrecedenceColumnGroup,
+  kBorderPrecedenceColumn,
+  kBorderPrecedenceRowGroup,
+  kBorderPrecedenceRow,
+  kBorderPrecedenceCell
+};
+
 class CollapsedBorderValue {
   DISALLOW_NEW_EXCEPT_PLACEMENT_NEW();
 
  public:
+  // Constructs a CollapsedBorderValue for non-existence border.
   CollapsedBorderValue()
       : color_(0),
         width_(0),
@@ -48,7 +59,9 @@ class CollapsedBorderValue {
         width_(border.NonZero() ? border.Width() : 0),
         style_(border.Style()),
         precedence_(precedence),
-        transparent_(border.IsTransparent()) {}
+        transparent_(border.IsTransparent()) {
+    DCHECK(precedence != kBorderPrecedenceOff);
+  }
 
   unsigned Width() const { return style_ > kBorderStyleHidden ? width_ : 0; }
   EBorderStyle Style() const { return static_cast<EBorderStyle>(style_); }
@@ -71,9 +84,7 @@ class CollapsedBorderValue {
            IsSameIgnoringColor(o);
   }
 
-  bool IsVisible() const {
-    return Style() > kBorderStyleHidden && !IsTransparent() && Exists();
-  }
+  bool IsVisible() const { return Width() && !IsTransparent(); }
 
   bool ShouldPaint(
       const CollapsedBorderValue& table_current_border_value) const {

third_party/WebKit/Source/core/layout/LayoutTable.h

@@ -29,8 +29,8 @@
 #include <memory>
 #include "core/CSSPropertyNames.h"
 #include "core/CoreExport.h"
+#include "core/layout/CollapsedBorderValue.h"
 #include "core/layout/LayoutBlock.h"
-#include "core/style/CollapsedBorderValue.h"
 #include "platform/wtf/Vector.h"
 
 namespace blink {

third_party/WebKit/Source/core/layout/LayoutTableCell.cpp

@@ -29,13 +29,13 @@
 #include "core/css/StylePropertySet.h"
 #include "core/editing/EditingUtilities.h"
 #include "core/html/HTMLTableCellElement.h"
+#include "core/layout/CollapsedBorderValue.h"
 #include "core/layout/LayoutAnalyzer.h"
 #include "core/layout/LayoutTableCol.h"
 #include "core/layout/SubtreeLayoutScope.h"
 #include "core/paint/ObjectPaintInvalidator.h"
 #include "core/paint/PaintLayer.h"
 #include "core/paint/TableCellPainter.h"
-#include "core/style/CollapsedBorderValue.h"
 #include "platform/geometry/FloatQuad.h"
 #include "platform/geometry/TransformState.h"
 #include "platform/wtf/PtrUtil.h"

third_party/WebKit/Source/core/paint/TableCellPainter.h

@@ -5,7 +5,6 @@
 #ifndef TableCellPainter_h
 #define TableCellPainter_h
 
-#include "core/style/CollapsedBorderValue.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/wtf/Allocator.h"
 

third_party/WebKit/Source/core/paint/TablePainter.cpp

@@ -4,9 +4,9 @@
 
 #include "core/paint/TablePainter.h"
 
+#include "core/layout/CollapsedBorderValue.h"
 #include "core/layout/LayoutTable.h"
 #include "core/layout/LayoutTableSection.h"
-#include "core/style/CollapsedBorderValue.h"
 #include "core/paint/BoxClipper.h"
 #include "core/paint/BoxPainter.h"
 #include "core/paint/LayoutObjectDrawingRecorder.h"

third_party/WebKit/Source/core/style/BUILD.gn

@@ -21,7 +21,6 @@ blink_core_sources("rendering") {
     "CachedUAStyle.h",
     "ClipPathOperation.cpp",
     "ClipPathOperation.h",
-    "CollapsedBorderValue.h",
     "ComputedStyle.cpp",
     "ComputedStyle.h",
     "ComputedStyleConstants.h",

third_party/WebKit/Source/core/style/ComputedStyleConstants.h

@@ -106,16 +106,6 @@ enum EBorderStyle {
   kBorderStyleDouble
 };
 
-enum EBorderPrecedence {
-  kBorderPrecedenceOff,
-  kBorderPrecedenceTable,
-  kBorderPrecedenceColumnGroup,
-  kBorderPrecedenceColumn,
-  kBorderPrecedenceRowGroup,
-  kBorderPrecedenceRow,
-  kBorderPrecedenceCell
-};
-
 enum OutlineIsAuto { kOutlineIsAutoOff = 0, kOutlineIsAutoOn };
 
 enum EMarginCollapse {