WebAuthn UI: Do not show back button on transport selection screen.

Bug: 849323
Change-Id: I2383118ba5f8032f0443c315972c890683a71f81
Reviewed-on: https://chromium-review.googlesource.com/1197366Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587712}

chrome/browser/ui/webauthn/sheet_models.cc

@@ -128,6 +128,10 @@ void AuthenticatorWelcomeSheetModel::OnAccept() {
 
 // AuthenticatorTransportSelectorSheetModel -----------------------------------
 
+bool AuthenticatorTransportSelectorSheetModel::IsBackButtonVisible() const {
+  return false;
+}
+
 gfx::ImageSkia* AuthenticatorTransportSelectorSheetModel::GetStepIllustration()
     const {
   return GetImage(IDR_WEBAUTHN_ILLUSTRATION_WELCOME);

chrome/browser/ui/webauthn/sheet_models.h

@@ -89,6 +89,7 @@ class AuthenticatorTransportSelectorSheetModel
 
  private:
   // AuthenticatorSheetModelBase:
+  bool IsBackButtonVisible() const override;
   gfx::ImageSkia* GetStepIllustration() const override;
   base::string16 GetStepTitle() const override;
   base::string16 GetStepDescription() const override;

chrome/browser/webauthn/authenticator_request_dialog_model.cc

@@ -237,11 +237,7 @@ void AuthenticatorRequestDialogModel::Cancel() {
 }
 
 void AuthenticatorRequestDialogModel::Back() {
-  if (current_step() == Step::kTransportSelection) {
-    Cancel();
-  } else {
-    SetCurrentStep(Step::kTransportSelection);
-  }
+  SetCurrentStep(Step::kTransportSelection);
 }
 
 void AuthenticatorRequestDialogModel::OnSheetModelDidChange() {