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

content/public/browser/download_interrupt_reason_values.h

 // Copyright (c) 2011 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.
 
 // Note that the embedder is welcome to persist these values across
 // invocations of the browser, and possibly across browser versions.
 // Thus individual errors may be deprecated and new errors added, but
 // the values of particular errors should not be changed.
 
 // File errors.
@@ skipping 68 matching lines @@
 // Server responses.
 
 // The server indicates that the operation has failed (generic).
 // "Server Error".
 INTERRUPT_REASON(SERVER_FAILED, 30)
 
 // The server does not support range requests.
 // Internal use only:  must restart from the beginning.
 INTERRUPT_REASON(SERVER_NO_RANGE, 31)
 
-// The download request does not meet the specified precondition.
-// Internal use only:  the file has changed on the server.
-INTERRUPT_REASON(SERVER_PRECONDITION, 32)
+// Precondition failed. This type of interruption could legitimately occur if a
+// partial download resumption was attempted using a If-Match header. However,
+// the downloads logic no longer uses If-Match headers and instead uses If-Range
+// headers where a precondition failure is not expected.
+//
+// Obsolete: INTERRUPT_REASON(SERVER_PRECONDITION, 32)
 
 // The server does not have the requested data.
 // "Unable to get file".
 INTERRUPT_REASON(SERVER_BAD_CONTENT, 33)
 
 // Server didn't authorize access to resource.
 INTERRUPT_REASON(SERVER_UNAUTHORIZED, 34)
 
 // Server certificate problem.
 INTERRUPT_REASON(SERVER_CERT_PROBLEM, 35)
@@ skipping 10 matching lines @@
 // The user shut down the browser.
 // Internal use only:  resume pending downloads if possible.
 INTERRUPT_REASON(USER_SHUTDOWN, 41)
 
 
 // Crash.
 
 // The browser crashed.
 // Internal use only:  resume pending downloads if possible.
 INTERRUPT_REASON(CRASH, 50)