Implement link selection on alt+mouse drag.

third_party/WebKit/Source/core/editing/SelectionController.h

Index: third_party/WebKit/Source/core/editing/SelectionController.h
diff --git a/third_party/WebKit/Source/core/editing/SelectionController.h b/third_party/WebKit/Source/core/editing/SelectionController.h
index eb259ec8053f3831e6c177e8ffe9ce44b98fdfc7..ea9104f5dc70bc7c3a109468cb39f8a665aaea6b 100644
--- a/third_party/WebKit/Source/core/editing/SelectionController.h
+++ b/third_party/WebKit/Source/core/editing/SelectionController.h
@@ -65,6 +65,7 @@ public:
     bool mouseDownMayStartSelect() const;
     bool mouseDownWasSingleClickInSelection() const;
     void notifySelectionChanged();
+    bool hasExtendedSelection() const { return m_selectionState == SelectionState::ExtendedSelection; }
 
 private:
     explicit SelectionController(LocalFrame&);
@@ -88,6 +89,8 @@ private:
     SelectionState m_selectionState;
 };
 
+bool isLinkSelection(const MouseEventWithHitTestResults&);
+
 } // namespace blink
 
 #endif // SelectionController_h