Add --use-more-webui runtime flag to toggle WebUI replacements for native dialogs.

chrome/chrome_browser.gypi

Index: chrome/chrome_browser.gypi
diff --git a/chrome/chrome_browser.gypi b/chrome/chrome_browser.gypi
index b4c1ad8d961bb7a9b93091fcab87ab2d49e8b8a7..1c87d8fb5a776a6fb17b4f4ec47d650243c495a2 100644
--- a/chrome/chrome_browser.gypi
+++ b/chrome/chrome_browser.gypi
@@ -3933,14 +3933,9 @@
             ],
           },
         }],
-        ['webui_dialogs == 1', {
-          'defines': [
-            'WEBUI_DIALOGS',
-          ],
-        }],
-        # Exclude WebUI certificate viewer if not POSIX, mac (these OS's have
-        # native certificate viewers) or WebUI dialogs are disabled.
-        ['webui_dialogs == 0 or os_posix == 0 or OS == "mac"', {
+        # Exclude WebUI certificate viewer if not POSIX or mac (these OS's have
+        # native certificate viewers).
+        ['os_posix == 0 or OS == "mac"', {
           'sources/': [
             ['exclude', '^browser/ui/webui/certificate_viewer.cc'],
             ['exclude', '^browser/ui/webui/certificate_viewer.h'],
@@ -3948,12 +3943,6 @@
             ['exclude', '^browser/ui/webui/certificate_viewer_ui.h'],
           ],
         }],
-        ['webui_dialogs == 1', {
-          'sources/': [
-            ['exclude', '^browser/ui/gtk/certificate_viewer.cc'],
-            ['exclude', '^browser/ui/gtk/certificate_viewer.h'],
-          ],
-        }],
         ['toolkit_uses_gtk == 1', {
           'dependencies': [
             '../build/linux/system.gyp:dbus-glib',
@@ -4599,14 +4588,6 @@
                 ['exclude', '^browser/ui/panels/panel_browser_window_gtk.h'],
               ],
             }],
-            # Exclude the GTK cert viewer again if webui_dialogs are
-            # enabled.
-            ['webui_dialogs==1', {
-              'sources/': [
-                ['exclude', '^browser/ui/gtk/certificate_viewer.cc'],
-                ['exclude', '^browser/ui/gtk/certificate_viewer.h'],
-              ],
-            }],
             # Exclude these toolkit_views specific files again.
             # (Required because of the '^browser/extensions/' include above)
             ['toolkit_views==0', {