fix getFavcion api in webview to return dataUrl

atom/renderer/lib/web-view/web-view-attributes.coffee

@@ -158,9 +158,8 @@ class SrcAttribute extends WebViewAttribute
     domEvent = new Event('will-navigate')
     domEvent['url'] = @getValue()
     domEvent.cancelable = true
-    self = @
-    domEvent.preventDefault = () ->
-      self.setValueIgnoreMutation ''
+    domEvent.preventDefault = () =>
+      @setValueIgnoreMutation ''
     @webViewImpl.webviewNode.dispatchEvent domEvent
 
     unless @webViewImpl.guestInstanceId?

atom/renderer/lib/web-view/web-view.coffee

@@ -236,7 +236,6 @@ registerWebViewElement = ->
   methods = [
     "getUrl"
     "getTitle"
-    "getFavicon"
     "isLoading"
     "isWaitingForResponse"
     "stop"
@@ -277,6 +276,13 @@ registerWebViewElement = ->
       remote.getGuestWebContents(internal.guestInstanceId)[m]  args...
   proto[m] = createHandler m for m in methods
 
+  # Return dataUrl instead of nativeImage.
+  proto.getFavicon = (args...) ->
+    internal = v8Util.getHiddenValue this, 'internal'
+    return unless internal
+    favicon = remote.getGuestWebContents(internal.guestInstanceId)['getFavicon'] args...
+    favicon.toDataUrl()
+
   window.WebView = webFrame.registerEmbedderCustomElement 'webview',
     prototype: proto