Convert LineEndpointComputationMode to template parameter in StartPositionForLine()

This patch changes |LineEndpointComputationMode| parameter to template parameter
in |StartPositionForLine()| to avoid runtime checking of parameter and reducing
source code size of body of |StartPositionForLine()| for improving readability.

The patch[1] will do same thing for |EndPositionForLine()| and remove
|LineEndpointComputationMode| enum.

[1] http://crrev.com/c/882531 Convert Li

Change-Id: I2345ff86d92423ba4dcf6852ec49b34e9563b31e
Reviewed-on: https://chromium-review.googlesource.com/882791
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531776}

third_party/WebKit/Source/core/editing/VisibleUnitsLine.cpp

@@ -174,10 +174,9 @@ InlineBox* FindRightNonPseudoNodeInlineBox(const RootInlineBox& root_box) {
 }
 
 enum LineEndpointComputationMode { kUseLogicalOrdering, kUseInlineBoxOrdering };
-template <typename Strategy>
+template <typename Strategy, typename Ordering>
 PositionWithAffinityTemplate<Strategy> StartPositionForLine(
-    const PositionWithAffinityTemplate<Strategy>& c,
-    LineEndpointComputationMode mode) {
+    const PositionWithAffinityTemplate<Strategy>& c) {
   if (c.IsNull())
     return PositionWithAffinityTemplate<Strategy>();
 
@@ -195,30 +194,47 @@ PositionWithAffinityTemplate<Strategy> StartPositionForLine(
     return PositionWithAffinityTemplate<Strategy>();
   }
 
-  Node* start_node;
-  InlineBox* start_box;
-  if (mode == kUseLogicalOrdering) {
-    start_node = root_box->GetLogicalStartBoxWithNode(start_box);
+  const auto& node_and_box = Ordering::StartNodeAndBoxOf(*root_box);
+  Node* const start_node = std::get<Node*>(node_and_box);
+  InlineBox* const start_box = std::get<InlineBox*>(node_and_box);
+  if (!start_node)
+    return PositionWithAffinityTemplate<Strategy>();
+  return PositionWithAffinityTemplate<Strategy>(
+      start_node->IsTextNode()
+          ? PositionTemplate<Strategy>(ToText(start_node),
+                                       ToInlineTextBox(start_box)->Start())
+          : PositionTemplate<Strategy>::BeforeNode(*start_node));
+}
+
+// Provides start and end of line in logical order for implementing Home and End
+// keys.
+struct LogicalOrdering {
+  static std::pair<Node*, InlineBox*> StartNodeAndBoxOf(
+      const RootInlineBox& root_box) {
+    InlineBox* start_box;
+    Node* const start_node = root_box.GetLogicalStartBoxWithNode(start_box);
     if (!start_node)
-      return PositionWithAffinityTemplate<Strategy>();
-  } else {
+      return {nullptr, nullptr};
+    return {start_node, start_box};
+  }
+};
+
+// Provides start end end of line in visual order for implementing expanding
+// selection in line granularity.
+struct VisualOrdering {
+  static std::pair<Node*, InlineBox*> StartNodeAndBoxOf(
+      const RootInlineBox& root_box) {
     // Generated content (e.g. list markers and CSS :before and :after
     // pseudoelements) have no corresponding DOM element, and so cannot be
     // represented by a VisiblePosition. Use whatever follows instead.
     // TODO(editing-dev): We should consider text-direction of line to
     // find non-pseudo node.
-    start_box = FindRightNonPseudoNodeInlineBox(*root_box);
+    InlineBox* const start_box = FindRightNonPseudoNodeInlineBox(root_box);
     if (!start_box)
-      return PositionWithAffinityTemplate<Strategy>();
-    start_node = start_box->GetLineLayoutItem().NonPseudoNode();
+      return {nullptr, nullptr};
+    return {start_box->GetLineLayoutItem().NonPseudoNode(), start_box};
   }
-
-  return PositionWithAffinityTemplate<Strategy>(
-      start_node->IsTextNode()
-          ? PositionTemplate<Strategy>(ToText(start_node),
-                                       ToInlineTextBox(start_box)->Start())
-          : PositionTemplate<Strategy>::BeforeNode(*start_node));
-}
+};
 
 template <typename Strategy>
 PositionWithAffinityTemplate<Strategy> StartOfLineAlgorithm(
@@ -226,7 +242,7 @@ PositionWithAffinityTemplate<Strategy> StartOfLineAlgorithm(
   // TODO: this is the current behavior that might need to be fixed.
   // Please refer to https://bugs.webkit.org/show_bug.cgi?id=49107 for detail.
   PositionWithAffinityTemplate<Strategy> vis_pos =
-      StartPositionForLine(c, kUseInlineBoxOrdering);
+      StartPositionForLine<Strategy, VisualOrdering>(c);
   return HonorEditingBoundaryAtOrBefore(vis_pos, c.GetPosition());
 }
 
@@ -351,7 +367,7 @@ static PositionWithAffinityTemplate<Strategy> LogicalStartOfLineAlgorithm(
   // TODO: this is the current behavior that might need to be fixed.
   // Please refer to https://bugs.webkit.org/show_bug.cgi?id=49107 for detail.
   PositionWithAffinityTemplate<Strategy> vis_pos =
-      StartPositionForLine(c, kUseLogicalOrdering);
+      StartPositionForLine<Strategy, LogicalOrdering>(c);
 
   if (ContainerNode* editable_root = HighestEditableRoot(c.GetPosition())) {
     if (!editable_root->contains(