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::GetManifest(const GetManifestCallback& callback) {
  manifest_manager_host_->GetManifest(GetMainFrame(), callback);
}

void WebContentsImpl::GetManifest(const GetManifestCallback& callback) {
  manifest_manager_host_->GetManifest(GetMainFrame(), callback);
}
