Add new API for only moving the selection extent point.

public/web/WebLocalFrame.h

Index: public/web/WebLocalFrame.h
diff --git a/public/web/WebLocalFrame.h b/public/web/WebLocalFrame.h
index e92eddc1dba82affa84e7e5aa2af4d7413728d91..826fdfa7fc8743068e2edbbd1fc23a13497948e0 100644
--- a/public/web/WebLocalFrame.h
+++ b/public/web/WebLocalFrame.h
@@ -67,6 +67,14 @@ public:
     // Associates an isolated world with human-readable name which is useful for
     // extension debugging.
     virtual void setIsolatedWorldHumanReadableName(int worldID, const WebString&) = 0;
+
+
+    // Selection --------------------------------------------------------------
+
+    // Moves the selection extent point. This function does not allow the
+    // selection to collapse. If the new extent is set to the same position as
+    // the current base, this function will do nothing.
+    virtual void moveRangeSelectionExtent(const WebPoint&) = 0;
 };
 
 } // namespace blink