Propagate the SafeBrowsing download protection verdict to the DownloadItem.

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
           "tab": {"$ref": "Tab", "optional": true, "description":"This property will <b>only</b> be present when the connection was opened from a tab or content script."},
@@ skipping 4297 matching lines @@
         "description": "The file has been determined to present no known danger to the user's computer."
       },
       "DANGER_FILE": {
         "type": "string",
         "value": "file",
         "description": "The download's filename is suspicious."
       },
       "DANGER_URL": {
         "type": "string",
         "value": "url",
-        "description": "The download's URL is suspicious."
+        "description": "The download's URL is known to be malicious."
+      },
+      "DANGER_CONTENT": {
+        "type": "string",
+        "value": "content",
+        "description": "The download's content is malicious."
       }
     },
     "types": [
       {
         "id": "DownloadItem",
         "type": "object",
         "description": "The state of a file as it is downloaded from the internet.",
         "properties": {
           "id": {
             "type": "integer",
@@ skipping 4879 matching lines @@
                   }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]