[Downloads] Rework how hashes are calculated for download files.

content/browser/download/download_item_impl_delegate.cc

Index: content/browser/download/download_item_impl_delegate.cc
diff --git a/content/browser/download/download_item_impl_delegate.cc b/content/browser/download/download_item_impl_delegate.cc
index c1ed48cd1248091464ed46a164c58ca76e3fc4bb..be7432599e18ee1fdbc0dd7aeac3c501b187c368 100644
--- a/content/browser/download/download_item_impl_delegate.cc
+++ b/content/browser/download/download_item_impl_delegate.cc
@@ -56,6 +56,11 @@ bool DownloadItemImplDelegate::ShouldOpenFileBasedOnExtension(
 void DownloadItemImplDelegate::CheckForFileRemoval(
     DownloadItemImpl* download_item) {}
 
+std::string DownloadItemImplDelegate::GetApplicationClientIdForFileScanning()
+    const {
+  return std::string();
+}
+
 void DownloadItemImplDelegate::ResumeInterruptedDownload(
     scoped_ptr<DownloadUrlParameters> params,
     uint32_t id) {}