Add CompositionMarkerList in preparation for DocumentMarkerController refactor

third_party/WebKit/Source/core/editing/markers/DocumentMarker.h

Index: third_party/WebKit/Source/core/editing/markers/DocumentMarker.h
diff --git a/third_party/WebKit/Source/core/editing/markers/DocumentMarker.h b/third_party/WebKit/Source/core/editing/markers/DocumentMarker.h
index dbcfdf2964555be13aa50b7551bf2305f37a6fd8..491ea4e401f73541d6721743efb620fe2e5ed623 100644
--- a/third_party/WebKit/Source/core/editing/markers/DocumentMarker.h
+++ b/third_party/WebKit/Source/core/editing/markers/DocumentMarker.h
@@ -161,8 +161,18 @@ class CORE_EXPORT DocumentMarker : public GarbageCollected<DocumentMarker> {
   void setActiveMatch(bool);
   void clearDetails() { m_details.clear(); }
 
-  // Offset modifications are done by DocumentMarkerController.
-  // Other classes should not call following setters.
+  struct ShiftMarkerResult {
+    unsigned newStartOffset;
+    unsigned newEndOffset;
+    bool shouldRemoveMarker;
+  };
+
+  ShiftMarkerResult getShiftedMarkerPosition(unsigned offset,

[Xiaocheng, 2017/03/25 00:05:35]

I think we should add unit test for this function.

Then unit tests for DML::shiftMarkers can be simplified:
1. Verify return value when nothing is modified
2. Verify return value and resulting list when a marker is modified
3. Verify return value and resulting list when a marker is removed

+                                             unsigned oldLength,
+                                             unsigned newLength) const;
+
+  // Offset modifications are done by DocumentMarkerController and subclasses of
+  // DocumentMarkerList. Other classes should not call following setters.
   void setStartOffset(unsigned offset) { m_startOffset = offset; }
   void setEndOffset(unsigned offset) { m_endOffset = offset; }
   void shiftOffsets(int delta);