Add hideKeyboard API to chrome.experimental.input.

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 2524 matching lines @@
         "type": "function",
         "description": "Notify input method engine page down button was clicked.",
         "parameters": [
           { "type": "function",
             "name": "callback",
             "optional": true,
             "description": "This function is called when the event processing is completed.",
             "parameters": []
           }
         ]
+      },
+      {
+        "name": "hideKeyboard",
+        "type": "function",
+        "description": "Hide the keyboard UI.",
+        "parameters": [
+          { "type": "function",
+            "name": "callback",
+            "optional": true,
+            "description": "This function is called when the event processing is completed.",
+            "parameters": []
+          }
+        ]
       }
     ],
     "events": [
       {
         "name": "onSetCursorLocation",
         "type": "function",
         "description": "Fired when input cursor location is changed.",
         "parameters": [
           { "type": "integer",
             "name": "x"
@@ skipping 3002 matching lines @@
           {
             "type": "integer",
             "name": "tabId",
             "description": "The id of the tab that was detached."
           }
         ]
       }
     ]
   }
 ]