browser: Abstract message box dialog functions into its own header file.

chrome/chrome_browser.gypi

Index: chrome/chrome_browser.gypi
diff --git a/chrome/chrome_browser.gypi b/chrome/chrome_browser.gypi
index c82f7667d822a5ed2711acad7d72ac9d59c7a007..dc5f742bdd60a50a4aaa2db605ad480ebe780409 100644
--- a/chrome/chrome_browser.gypi
+++ b/chrome/chrome_browser.gypi
@@ -1981,6 +1981,7 @@
         'browser/sidebar/sidebar_container.h',
         'browser/sidebar/sidebar_manager.cc',
         'browser/sidebar/sidebar_manager.h',
+        'browser/simple_message_box.h',
         'browser/speech/speech_input_bubble.cc',
         'browser/speech/speech_input_bubble.h',
         'browser/speech/speech_input_bubble_controller.cc',
@@ -2608,6 +2609,7 @@
         'browser/ui/cocoa/shell_dialogs_mac.mm',
         'browser/ui/cocoa/sidebar_controller.h',
         'browser/ui/cocoa/sidebar_controller.mm',
+        'browser/ui/cocoa/simple_message_box_mac.mm',
         'browser/ui/cocoa/speech_input_window_controller.h',
         'browser/ui/cocoa/speech_input_window_controller.mm',
         'browser/ui/cocoa/ssl_client_certificate_selector.mm',
@@ -2908,6 +2910,7 @@
         'browser/ui/gtk/rounded_window.h',
         'browser/ui/gtk/sad_tab_gtk.cc',
         'browser/ui/gtk/sad_tab_gtk.h',
+        'browser/ui/gtk/simple_message_box_gtk.cc',
         'browser/ui/gtk/slide_animator_gtk.cc',
         'browser/ui/gtk/slide_animator_gtk.h',
         'browser/ui/gtk/ssl_client_certificate_selector.cc',
@@ -3304,6 +3307,7 @@
         'browser/ui/views/sad_tab_view.h',
         'browser/ui/views/select_file_dialog.cc',
         'browser/ui/views/shell_dialogs_win.cc',
+        'browser/ui/views/simple_message_box_win.cc',
         'browser/ui/views/ssl_client_certificate_selector.cc',
         'browser/ui/views/ssl_client_certificate_selector.h',
         'browser/ui/views/status_bubble_views.cc',
@@ -4279,6 +4283,7 @@
                 ['include', '^browser/ui/gtk/owned_widget_gtk.h'],
                 ['include', '^browser/ui/gtk/repost_form_warning_gtk.cc'],
                 ['include', '^browser/ui/gtk/repost_form_warning_gtk.h'],
+                ['include', '^browser/ui/gtk/simple_message_box_gtk.cc'],
                 ['include', '^browser/ui/gtk/ssl_client_certificate_selector.cc'],
                 ['include', '^browser/ui/gtk/tab_contents_drag_source.cc'],
                 ['include', '^browser/ui/gtk/tab_contents_drag_source.h'],
@@ -4629,6 +4634,7 @@
               'sources/': [
                 ['include', '^browser/ui/gtk/omnibox/omnibox_view_gtk.cc'],
                 ['include', '^browser/ui/gtk/omnibox/omnibox_view_gtk.h'],
+                ['include', '^browser/ui/gtk/simple_message_box_gtk.cc'],
                 ['include', '^browser/ui/login/login_prompt_ui.cc'],
                 ['include', '^browser/ui/views/omnibox/omnibox_view_views.cc'],
                 ['include', '^browser/ui/views/omnibox/omnibox_view_views.h'],