Fix filter exception when injecting CSS (PR #837)

When injecting CSS, an `onBeforeRequest paramater 'filter' must have property 'urls'` error is thrown.

![image](https://user-images.githubusercontent.com/599164/63574823-a3dd6180-c5c3-11e9-89c0-a9debdd4f696.png)

This is caused by [Electron PR #19337: Throw on invalid webRequest filters](https://github.com/electron/electron/pull/19337).

So, pass filter argument properly, like Electron expects it strictly.

app/src/components/mainWindow/mainWindow.js

@@ -58,7 +58,7 @@ function maybeInjectCss(browserWindow) {
     // we have to inject the css in onHeadersReceived so they're early enough
     // will run multiple times, so did-finish-load will remove this handler
     browserWindow.webContents.session.webRequest.onHeadersReceived(
-      [], // Pass an empty filter list; null will not match _any_ urls
+      { urls: [] }, // Pass an empty filter list; null will not match _any_ urls
       onHeadersReceived,
     );
   });