Update touch selection to only modify one selection point at a time.

content/common/input_messages.h

Index: content/common/input_messages.h
diff --git a/content/common/input_messages.h b/content/common/input_messages.h
index 1827212699c9acb9e3283ddd7f975834ee66f34f..0a95dac59fab5e8cbcb977e86b8e74f02934649d 100644
--- a/content/common/input_messages.h
+++ b/content/common/input_messages.h
@@ -202,6 +202,10 @@ IPC_MESSAGE_ROUTED2(InputMsg_SelectRange,
                     gfx::Point /* start */,
                     gfx::Point /* end */)
 
+// Requests the renderer to move the selection extent point to a new position.
+IPC_MESSAGE_ROUTED1(InputMsg_MoveSelectionExtent,
+                    gfx::Point /* point */)
+
 // Requests the renderer to move the caret selection toward the point.
 // Expects a MoveCaret_ACK message when finished.
 IPC_MESSAGE_ROUTED1(InputMsg_MoveCaret,