startup: make gaia credential dialog window-modal

This dialog is shown on the winlogon desktop to prompt for Google
credentials to use for Windows login. It appears in testing to
still work properly when window-modal.

Bug: 1121132
Change-Id: Ibb611ef58fafb14948f79173211e29eddf678584
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405701Reviewed-by: Yusuf Sengul <yusufsn@google.com>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806657}

chrome/browser/ui/startup/credential_provider_signin_dialog_win.cc

@@ -317,7 +317,7 @@ class CredentialProviderWebDialogDelegate : public ui::WebDialogDelegate {
   }
 
   ui::ModalType GetDialogModalType() const override {
-    return ui::MODAL_TYPE_SYSTEM;
+    return ui::MODAL_TYPE_WINDOW;
   }
 
   base::string16 GetDialogTitle() const override { return base::string16(); }