Don't use http cache for offline page download.

Most DownloadManager::DownloadUrl use cases don't use cache because
cache can result in download with a partial file, and it can block
video thumbnail generation when the partial file is too small.

Bug: 909605
Change-Id: I40c8dae00da677d969f1a3c398b758d4e1d230a4
Reviewed-on: https://chromium-review.googlesource.com/c/1357501Reviewed-by: Jian Li <jianli@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612807}

chrome/browser/offline_pages/android/downloads/offline_page_download_bridge.cc

@@ -287,8 +287,6 @@ void DownloadAsFile(content::WebContents* web_contents, const GURL& url) {
   dl_params->set_referrer(referrer.url);
   dl_params->set_referrer_policy(
       content::Referrer::ReferrerPolicyForUrlRequest(referrer.policy));
-
-  dl_params->set_prefer_cache(true);
   dl_params->set_prompt(false);
   dl_params->set_download_source(download::DownloadSource::OFFLINE_PAGE);
   dlm->DownloadUrl(std::move(dl_params));