[identity] getAuthToken(): inject JS bridge earlier

Sometimes OAuthConsent.setConsentResult() JS Bridge is called by the
server before it has been initialized by the injected JavaScript, so the
browser doesn’t receive the consent result.

This CL makes Chrome inject JS bridge as early as possible, after the
navigation commit.

Fixed: 1075932
Change-Id: I4030163cd51b8af6be785be577412b2d5f077687
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2169910Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763432}

chrome/browser/resources/identity_scope_approval_dialog/scope_approval_dialog.js

@@ -44,6 +44,8 @@ function loadAuthUrlAndShowWindow(url, win) {
       windowId = win.id;
       windowShown = true;
     }
+  });
+  webview.addEventListener('loadcommit', function() {
     webview.executeScript({file: 'inject.js'});
   });
 }