webauthn: hide back button from caBLE sheet on Windows.

When the native Windows API is in use, there's no point in being able
to go "back" to the transport selection sheet because the only entry
will be "Your phone". The native API can be triggered by the drop-down
list.

BUG=1002262

Change-Id: I4791542c9e18f5214deee211dc6afcac22cacdc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032012
Commit-Queue: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Auto-Submit: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#737111}

chrome/browser/ui/webauthn/sheet_models.cc

@@ -18,6 +18,7 @@
 #include "components/strings/grit/components_strings.h"
 #include "components/url_formatter/elide_url.h"
 #include "device/fido/authenticator_get_assertion_response.h"
+#include "device/fido/features.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/gfx/font_list.h"
 #include "ui/gfx/text_utils.h"
@@ -742,6 +743,14 @@ AuthenticatorPaaskSheetModel::AuthenticatorPaaskSheetModel(
 
 AuthenticatorPaaskSheetModel::~AuthenticatorPaaskSheetModel() = default;
 
+bool AuthenticatorPaaskSheetModel::IsBackButtonVisible() const {
+#if defined(OS_WIN)
+  return !base::FeatureList::IsEnabled(device::kWebAuthUseNativeWinApi);
+#else
+  return true;
+#endif
+}
+
 bool AuthenticatorPaaskSheetModel::IsActivityIndicatorVisible() const {
   return true;
 }

chrome/browser/ui/webauthn/sheet_models.h

@@ -354,6 +354,7 @@ class AuthenticatorPaaskSheetModel : public AuthenticatorSheetModelBase {
 
  private:
   // AuthenticatorSheetModelBase:
+  bool IsBackButtonVisible() const override;
   bool IsActivityIndicatorVisible() const override;
   const gfx::VectorIcon& GetStepIllustration(
       ImageColorScheme color_scheme) const override;