Fix implementation of the startup notification protocol for the main instance

At the moment, the implementation of the startup notification protocol [1]
does only make sure that the window manager gets notified when the startup
process finishes for non-primary instances of the main process, that is,
when the user attempts to spawn a new chromium-browser process while another
instance is already running.

This is because ProcessSingleton::NotifyOtherProcessWithTimeout() is the only
placed where NotifyWindowManagerStartupComplete() is being called, which used
to be fine when chromium relied on GTK+ for its top-level windows (GTK+ calls
gdk_display_notify_startup_complete() when mapping the top-level window) in
the past, but now that chromium relies on Aura, it's necessary to add another
explicit call in the codepath that completes startup for the main instance.

[1] https://www.freedesktop.org/wiki/Specifications/startup-notification-spec

Bug: 553956
Change-Id: Ie9bf2db9bc04589229a18785771b6fc97dbcc9bd
Reviewed-on: https://chromium-review.googlesource.com/1072088Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565390}

chrome/browser/after_startup_task_utils.cc

@@ -27,6 +27,10 @@
 #include "content/public/browser/web_contents_observer.h"
 #include "third_party/blink/public/mojom/page/page_visibility_state.mojom.h"
 
+#if defined(OS_LINUX) && !defined(OS_CHROMEOS)
+#include "ui/views/linux_ui/linux_ui.h"
+#endif
+
 using content::BrowserThread;
 using content::WebContents;
 using content::WebContentsObserver;
@@ -120,6 +124,14 @@ void SetBrowserStartupIsComplete() {
   // The shrink_to_fit() method is not available for all of our build targets.
   base::circular_deque<AfterStartupTask*>(g_after_startup_tasks.Get())
       .swap(g_after_startup_tasks.Get());
+
+#if defined(OS_LINUX) && !defined(OS_CHROMEOS)
+  // Make sure we complete the startup notification sequence, or launchers will
+  // get confused by not receiving the expected message from the main process.
+  views::LinuxUI* linux_ui = views::LinuxUI::instance();
+  if (linux_ui)
+    linux_ui->NotifyWindowManagerStartupComplete();
+#endif
 }
 
 // Observes the first visible page load and sets the startup complete

ui/views/linux_ui/linux_ui.h

@@ -165,9 +165,9 @@ class VIEWS_EXPORT LinuxUI : public ui::LinuxInputMethodContextFactory,
       NonClientWindowFrameActionSourceType source) = 0;
 
   // Notifies the window manager that start up has completed.
-  // Normally Chromium opens a new window on startup and GTK does this
-  // automatically. In case Chromium does not open a new window on startup,
-  // e.g. an existing browser window already exists, this should be called.
+  // This needs to be called explicitly both on the primary and the "remote"
+  // instances (e.g. an existing browser window already exists), since we no
+  // longer use GTK (which did this automatically) for the main windows.
   virtual void NotifyWindowManagerStartupComplete() = 0;
 
   // Updates the device scale factor so that the default font size can be