Set Cancel as the default button for the external protocol dialog.

This ensures that users holding down the Enter or Space key cannot
accidentally trigger an external protocol launch.

BUG=865202

Change-Id: I2cec7b3c216b80641200c97dec2517a66b4e0b24
Reviewed-on: https://chromium-review.googlesource.com/1142705
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576340}

chrome/browser/ui/views/external_protocol_dialog.cc

@@ -57,6 +57,10 @@ bool ExternalProtocolDialog::ShouldShowCloseButton() const {
   return false;
 }
 
+int ExternalProtocolDialog::GetDefaultDialogButton() const {
+  return ui::DIALOG_BUTTON_CANCEL;
+}
+
 base::string16 ExternalProtocolDialog::GetDialogButtonLabel(
     ui::DialogButton button) const {
   return delegate_->GetDialogButtonLabel(button);

chrome/browser/ui/views/external_protocol_dialog.h

@@ -32,6 +32,7 @@ class ExternalProtocolDialog : public views::DialogDelegateView {
   // views::DialogDelegateView:
   gfx::Size CalculatePreferredSize() const override;
   bool ShouldShowCloseButton() const override;
+  int GetDefaultDialogButton() const override;
   base::string16 GetDialogButtonLabel(ui::DialogButton button) const override;
   base::string16 GetWindowTitle() const override;
   bool Cancel() override;