Mitigate undesireable side-effect of 901c325eab

Related discussion/issue:
- https://github.com/uBlockOrigin/uMatrix-issues/issues/156#issuecomment-494427094
- https://github.com/uBlockOrigin/uMatrix-issues/issues/155

Due to a Chromium issue about not providing the proper context
information, a negative side-effect was introduced with fix
to #155. This commit will force the originator of a network
request of type `main_frame` to be that of the request URL
itself, i.e. the originator of a top-level document request
is the requested document itself.

platform/chromium/vapi-webrequest.js

@@ -71,17 +71,21 @@
     const parsedURL = new URL('https://www.example.org/');
 
     vAPI.net.normalizeDetails = function(details) {
+        let type = details.type;
+
+        // https://github.com/uBlockOrigin/uMatrix-issues/issues/156#issuecomment-494427094
+        if ( type === 'main_frame' ) {
+            details.documentUrl = details.url;
+        }
         // Chromium 63+ supports the `initiator` property, which contains
         // the URL of the origin from which the network request was made.
-        if (
+        else if (
             typeof details.initiator === 'string' &&
             details.initiator !== 'null'
         ) {
             details.documentUrl = details.initiator;
         }
 
-        let type = details.type;
-
         // https://github.com/gorhill/uBlock/issues/1493
         // Chromium 49+/WebExtensions support a new request type: `ping`,
         // which is fired as a result of using `navigator.sendBeacon`.