[Extensions UI] Initially disabled OK button for extension install prompts and enable them after a 500 ms time period.

BUG=394518

Review-Url: https://codereview.chromium.org/2716353003
Cr-Commit-Position: refs/heads/master@{#461933}
ExtensionInstallDialogView::~ExtensionInstallDialogView() {
  if (!handled_result_ && !done_callback_.is_null()) {
    base::ResetAndReturn(&done_callback_)
        .Run(ExtensionInstallPrompt::Result::USER_CANCELED);
  }
}

ExtensionInstallDialogView::~ExtensionInstallDialogView() {
  if (!handled_result_ && !done_callback_.is_null()) {
    base::ResetAndReturn(&done_callback_)
        .Run(ExtensionInstallPrompt::Result::USER_CANCELED);
  }
}
