fido: disable back button in Touch ID UI sheet

TouchIdAuthenticator currently fails a DCHECK when GetAssertion or
MakeCredential get invoked multiple times. Using the Back button (and
then proceeding to Touch ID again) can trigger a second call to
DispatchRequest and therefore the DCHECK fail.

Clicking the back button on the Touch ID sheet would not dismiss the
native Touch ID dialog, which would then hover over the welcome screen
sheet. So simply disabling the button seems like the right thing to do.

Bug: 678128,847985
Change-Id: I90dbf5ab016a177811575a4db61ca12d15e43841
Reviewed-on: https://chromium-review.googlesource.com/1184236
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584983}

chrome/browser/ui/webauthn/sheet_models.cc

@@ -382,6 +382,12 @@ bool AuthenticatorTouchIdSheetModel::IsActivityIndicatorVisible() const {
   return true;
 }
 
+bool AuthenticatorTouchIdSheetModel::IsBackButtonVisible() const {
+  // Clicking back would not dismiss the native Touch ID dialog, which would be
+  // confusing. The user can cancel the native dialog to dismiss it.
+  return false;
+}
+
 gfx::ImageSkia* AuthenticatorTouchIdSheetModel::GetStepIllustration() const {
 #if defined(OS_MACOSX)
   return GetImage(IDR_WEBAUTHN_ILLUSTRATION_TOUCHID_1X);

chrome/browser/ui/webauthn/sheet_models.h

@@ -225,6 +225,7 @@ class AuthenticatorTouchIdSheetModel : public AuthenticatorSheetModelBase {
  private:
   // AuthenticatorSheetModelBase:
   bool IsActivityIndicatorVisible() const override;
+  bool IsBackButtonVisible() const override;
   gfx::ImageSkia* GetStepIllustration() const override;
   base::string16 GetStepTitle() const override;
   base::string16 GetStepDescription() const override;