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/native/browser_platform_delegate_native_win.cc

@@ -10,7 +10,6 @@
 
 #include "libcef/browser/alloy/alloy_browser_host_impl.h"
 #include "libcef/browser/context.h"
-#include "libcef/browser/native/file_dialog_runner_win.h"
 #include "libcef/browser/native/javascript_dialog_runner_win.h"
 #include "libcef/browser/native/menu_runner_win.h"
 #include "libcef/browser/native/window_delegate_view.h"
@@ -447,11 +446,6 @@ CefEventHandle CefBrowserPlatformDelegateNativeWin::GetEventHandle(
       const_cast<CHROME_MSG*>(&event.os_event->native_event()));
 }
 
-std::unique_ptr<CefFileDialogRunner>
-CefBrowserPlatformDelegateNativeWin::CreateFileDialogRunner() {
-  return base::WrapUnique(new CefFileDialogRunnerWin);
-}
-
 std::unique_ptr<CefJavaScriptDialogRunner>
 CefBrowserPlatformDelegateNativeWin::CreateJavaScriptDialogRunner() {
   return base::WrapUnique(new CefJavaScriptDialogRunnerWin);