Don't call WebContents::DownloadImage() callback if the WebContents were deleted

BUG=583718

Review URL: https://codereview.chromium.org/1685343004

Cr-Commit-Position: refs/heads/master@{#375700}
void WebContentsImpl::DidRedirectNavigation(
    NavigationHandle* navigation_handle) {
  FOR_EACH_OBSERVER(WebContentsObserver, observers_,
                    DidRedirectNavigation(navigation_handle));
}

void WebContentsImpl::DidRedirectNavigation(
    NavigationHandle* navigation_handle) {
  FOR_EACH_OBSERVER(WebContentsObserver, observers_,
                    DidRedirectNavigation(navigation_handle));
}
