Use If-Range instead of If-Match/If-Unmodified-Since for partial requests.

chrome/common/extensions/api/downloads.idl

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Use the <code>chrome.downloads</code> API to programmatically initiate,
 // monitor, manipulate, and search for downloads.
 [permissions=downloads]
 namespace downloads {
   [inline_doc] dictionary HeaderNameValuePair {
     // Name of the HTTP header.
@@ skipping 37 matching lines @@
     FILE_BLOCKED,
     FILE_SECURITY_CHECK_FAILED,
     FILE_TOO_SHORT,
     NETWORK_FAILED,
     NETWORK_TIMEOUT,
     NETWORK_DISCONNECTED,
     NETWORK_SERVER_DOWN,
     NETWORK_INVALID_REQUEST,
     SERVER_FAILED,
     SERVER_NO_RANGE,
-    SERVER_PRECONDITION,

[Devlin, 2015/12/02 17:18:27]

Unfortunately, we expose these enums on the api object, so this would have the
potential to break code if anyone used
chrome.downloads.InterruptReason.SERVER_PRECONDITION, this would break.

Luckily, it looks as though no one does, so this should be safe.  But just as
FYI that, in general, we shouldn't be doing this.

[asanka, 2015/12/02 17:23:52]

Yup. See comment #12 (relevant portion pasted below):

---
  Note that this change causes "SERVER_PRECONDITION" to no longer be recognized
as an InterruptReason. This affects search() and erase() calls which use a
DownloadQuery that specifies an 'error' field of 'SERVER_PRECONDITION'. This
shouldn't happen because the SERVER_PRECONDITION interrupt reason is an
ephemeral state during download resumption and was not expected to be set on a
download in a way that search() and erase() could've observed it. 

  I'm removing it because we will now never set this interrupt reason on a
download. In the longer term it would be great if we could remove the specific
interrupt reasons from the public API (probably infeasible) because they are
exposing implementation details in a way that is already causing problems.
---

     SERVER_BAD_CONTENT,
     SERVER_UNAUTHORIZED,
     SERVER_CERT_PROBLEM,
     SERVER_FORBIDDEN,
     USER_CANCELED,
     USER_SHUTDOWN,
     CRASH};
 
   [inline_doc] dictionary DownloadOptions {
     // The URL to download.
@@ skipping 474 matching lines @@
     // <code>suggestion</code> object to <code>suggest</code> wins. In order to
     // avoid confusion regarding which extension will win, users should not
     // install extensions that may conflict. If the download is initiated by
     // $(ref:download) and the target filename is known before the MIME type and
     // tentative filename have been determined, pass <code>filename</code> to
     // $(ref:download) instead.
     [maxListeners=1] static void onDeterminingFilename(
         DownloadItem downloadItem, SuggestFilenameCallback suggest);
   };
 };