Implement a poor man's PostAfterStartupTask() function.

content/public/browser/content_browser_client.cc

Index: content/public/browser/content_browser_client.cc
diff --git a/content/public/browser/content_browser_client.cc b/content/public/browser/content_browser_client.cc
index 67d2df7c1bf839247279e9a134aa3e7a52900f22..e0bf10b47ffbb650b63e4ef17e88b8037127c373 100644
--- a/content/public/browser/content_browser_client.cc
+++ b/content/public/browser/content_browser_client.cc
@@ -5,6 +5,7 @@
 #include "content/public/browser/content_browser_client.h"
 
 #include "base/files/file_path.h"
+#include "content/browser/after_startup_task_poster_impl.h"
 #include "ui/gfx/image/image_skia.h"
 #include "url/gurl.h"
 
@@ -15,6 +16,11 @@ BrowserMainParts* ContentBrowserClient::CreateBrowserMainParts(
   return nullptr;
 }
 
+AfterStartupTaskPoster* ContentBrowserClient::GetAfterStartupTaskPoster() {
+  static DefaultAfterStartupTaskPoster default_task_poster;
+  return &default_task_poster;
+}
+
 WebContentsViewDelegate* ContentBrowserClient::GetWebContentsViewDelegate(
     WebContents* web_contents) {
   return nullptr;