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::RouteCloseEvent(RenderViewHost* rvh) {
  if (rvh->GetSiteInstance()->IsRelatedSiteInstance(GetSiteInstance()))
    ClosePage();
}

void WebContentsImpl::RouteCloseEvent(RenderViewHost* rvh) {
  if (rvh->GetSiteInstance()->IsRelatedSiteInstance(GetSiteInstance()))
    ClosePage();
}
