Use Chrome file dialogs on all platforms and runtimes (fixes issue #3314)

All file dialogs irrespective of source, platform and runtime will now be
routed through CefFileDialogManager and trigger CefDialogHandler callbacks
(see issue #3293).

Adds Chrome runtime support for CefBrowserHost::RunFileDialog and
CefDialogHandler callbacks.

Adds Alloy runtime support for internal GTK file and print dialogs on Linux
subject to the following limitations:

1. Internal GTK implementation:
   - Cannot be used with multi-threaded-message-loop because Chromium's
     internal GTK implementation is not thread-safe (does not use GDK threads).
   - Dialogs will not be modal to application windows when used with off-screen
     rendering due to lack of access to the client's top-level GtkWindow.
2. Cefclient CefDialogHandler implementation:
   - Cannot be used with Views because it requires a top-level GtkWindow.

Due to the above limitations no dialog implementation is currently provided for
Views + multi-threaded-message-loop on Linux. In cases where both
implementations are supported the cefclient version is now behind an optional
`--use-client-dialogs` command-line flag.

Expressly forbids multiple simultaneous file dialogs with the internal platform
implementation which uses modal dialogs. CefDialogHandler will still be notified
and can optionally handle each request without a modal dialog (see issue #3154).

Removes some RunFileDialog parameters that are not supported by the Chrome file
dialog implementation (selected_accept_filter parameter, cef_file_dialog_mode_t
overwrite/read-only flags).

libcef/browser/devtools/devtools_file_manager.h

@@ -49,7 +49,6 @@ class CefDevToolsFileManager {
                              const std::string& content,
                              SaveCallback saveCallback,
                              CancelCallback cancelCallback,
-                             int selected_accept_filter,
                              const std::vector<base::FilePath>& file_paths);
   void SaveAsFileSelected(const std::string& url,
                           const std::string& content,