Add zoom extension API

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": {
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               {
                 "name": "tab",
                 "$ref": "Tab",
                 "description": "Details about the moved tab."
               }
             ]
           }
         ]
       },
       {
+        "name": "setZoomPercent",
+        "type": "function",
+        "description": "Set the zoom percentage.",
+        "parameters": [
+          {
+            "name": "tabId",
+            "type": "integer",
+            "minimum": 0,
+            "description": "The id of the tab to zoom."
+          },
+          {
+            "name": "zoomLevel",
+            "type": "number",
+            "description": "The zoom percentage."
+          },
+          {
+            "name": "callback",
+            "type": "function",
+            "optional": true,
+            "description": "Returns the resulting zoom percent, which may be different than requested.",
+            "parameters": [
+              {
+                "name": "zoomPercent",
+                "type": "number",
+                "description": "The current zoom percentage"
+              }
+            ]
+          }
+        ]
+      },
+      {
+        "name": "getZoomPercent",
+        "type": "function",
+        "description": "Get the zoom percentage.",
+        "parameters": [
+          {
+            "name": "tabId",
+            "type": "integer",
+            "minimum": 0,
+            "description": "The id of the tab."
+          },
+          {
+            "name": "callback",
+            "type": "function",
+            "description": "Returns the zoom percent.",
+            "parameters": [
+              {
+                "name": "zoomPercent",
+                "type": "number",
+                "description": "The current zoom percentage"
+              }
+            ]
+          }
+        ]
+      },
+      {
         "name": "remove",
         "type": "function",
         "description": "Closes a tab.",
         "parameters": [
           {"type": "integer", "name": "tabId", "minimum": 0},
           {"type": "function", "name": "callback", "optional": true, "parameters": []}
         ]
       },
       {
         "name": "detectLanguage",
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                   "token": { "type": "string", "optional": true }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]