Implementation of device metrics emulation in render view.

content/common/view_messages.h

Index: content/common/view_messages.h
===================================================================
--- content/common/view_messages.h	(revision 221161)
+++ content/common/view_messages.h	(working copy)
@@ -890,6 +890,12 @@
 // compositor path.
 IPC_MESSAGE_ROUTED0(ViewMsg_SwapBuffers_ACK)
 
+// Tells the render view that a ViewHostMsg_EmulateDevice message
+// was processed. This signals the render view that it's safe to emulate,
+// and host will accept rendered image of any size.
+IPC_MESSAGE_ROUTED1(ViewMsg_EmulateDevice_ACK,
+                    bool /* enabled */)
+
 // Tells the render widget that a smooth scroll completed.
 IPC_MESSAGE_ROUTED0(ViewMsg_SyntheticGestureCompleted)
 
@@ -1649,6 +1655,12 @@
 IPC_MESSAGE_ROUTED1(ViewHostMsg_BeginSmoothScroll,
                     ViewHostMsg_BeginSmoothScroll_Params /* params */)
 
+// Sent to initiate emulating device mode. In this mode, rendered image may be
+// not the same size as view itself. In response to this message, the host
+// generates a ViewMsg_EmulateDevice_ACK message.
+IPC_MESSAGE_ROUTED1(ViewHostMsg_EmulateDevice,
+                    bool /* enabled */)
+
 IPC_MESSAGE_ROUTED0(ViewHostMsg_Focus)
 IPC_MESSAGE_ROUTED0(ViewHostMsg_Blur)