Fix flymake to work with my Emacs, and hopefully not break others.

tools/emacs/flymake-chromium.el

 ;; 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.
 
 ;; Set up flymake for use with chromium code.  Uses ninja (since none of the
 ;; other chromium build systems have latency that allows interactive use).
 ;;
 ;; Requires a modern emacs (GNU Emacs >= 23) and that gyp has already generated
 ;; the build.ninja file(s).  See defcustoms below for settable knobs.
 
 
 (require 'flymake)
 
 (defcustom cr-flymake-ninja-build-file "out/Debug/build.ninja"
   "Relative path from chromium's src/ directory to the
   build.ninja file to use.")
 
 (defcustom cr-flymake-ninja-executable "ninja"
   "Ninja executable location; either in $PATH or explicitly given.")
 
+(defun cr-flymake-absbufferpath ()
+  "Return the absolute path to the current buffer, or nil if the
+  current buffer has no path."
+  (when buffer-file-truename
+      (expand-file-name buffer-file-truename)))
+
 (defun cr-flymake-chromium-src ()
   "Return chromium's src/ directory, or nil on failure."
-  (locate-dominating-file buffer-file-truename cr-flymake-ninja-build-file))
+  (let ((srcdir (locate-dominating-file
+                 (cr-flymake-absbufferpath) cr-flymake-ninja-build-file)))
+    (when srcdir (expand-file-name srcdir))))
 
 (defun cr-flymake-string-prefix-p (prefix str)
   "Return non-nil if PREFIX is a prefix of STR (23.2 has string-prefix-p but
   that's case insensitive and also 23.1 doesn't have it)."
   (string= prefix (substring str 0 (length prefix))))
 
 (defun cr-flymake-current-file-name ()
   "Return the relative path from chromium's src/ directory to the
   file backing the current buffer or nil if it doesn't look like
   we're under chromium/src/."
   (when (and (cr-flymake-chromium-src)
              (cr-flymake-string-prefix-p
-              (cr-flymake-chromium-src) buffer-file-truename))
-    (substring buffer-file-truename (length (cr-flymake-chromium-src)))))
+              (cr-flymake-chromium-src) (cr-flymake-absbufferpath)))
+    (substring (cr-flymake-absbufferpath) (length (cr-flymake-chromium-src)))))
 
 (defun cr-flymake-from-build-to-src-root ()
   "Return a path fragment for getting from the build.ninja file to src/."
   (replace-regexp-in-string
    "[^/]+" ".."
    (substring
     (file-name-directory
      (file-truename (or (and (cr-flymake-string-prefix-p
                               "/" cr-flymake-ninja-build-file)
                              cr-flymake-ninja-build-file)
@@ skipping 57 matching lines @@
       (kill-local-variable 'flymake-allowed-file-name-masks))))
 
 (add-hook 'find-file-hook 'cr-flymake-find-file 'append)
 (add-hook 'after-save-hook 'cr-flymake-kick-off-check-after-save)
 
 ;; Show flymake infrastructure ERRORs in hopes of fixing them.  Set to 3 for
 ;; DEBUG-level output from flymake.el.
 (setq flymake-log-level 0)
 
 (provide 'flymake-chromium)