Use process/control messages (instead of RVH messages) for layout tests.

third_party/WebKit/LayoutTests/inspector-protocol/accessibility/accessibility-nameSources-summary-expected.txt

+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
+CONSOLE ERROR: line 1: Uncaught TypeError: DevToolsAPI.embedderMessageAck is not a function
 {
   "id": "<int>",
   "result": {
     "accessibilityNode": {
       "nodeId": "<string>",
       "ignored": false,
       "role": {
         "type": "internalRole",
         "value": "DisclosureTriangle"
       },
@@ skipping 243 matching lines @@
                 "nodeResult": "span#labelledby5"
               }
             ]
           }
         }
       ]
     }
   }
 }