Force use_ash to 0 if use_aura is 0.

build/common.gypi

 # 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 53 matching lines @@
           # based on 'buildtype' (i.e. we don't care about saving symbols for
           # non-Official # builds).
           'buildtype%': 'Dev',
 
           # Override branding to select the desired branding flavor.
           'branding%': 'Chromium',
 
           'conditions': [
             # ChromeOS and Windows use Aura and Ash.
             ['chromeos==1 or OS=="win"', {
               'use_ash%': 1,

[scottmg, 2013/10/03 19:15:42]

i don't think subsequent conditions at the same level "do what you want"? but i
could be wrong.. if it works, lgtm.

[gab, 2013/10/03 19:48:35]

Ya, I'm also surprised that this works (I just proved it to myself by popping a
message box inside a non-aura ifdef from browser main)...

It seems values explicitly specified in chromium.gyp_env are not overridden by
whatever gyp says.
I just "proved" this to myself by trying to specify "use_aura=0 use_ash=1" in my
chromium.gyp_env and noticing that the new rule I added below didn't have any
effect (i.e. use_ash remained 1).

               'use_aura%': 1,
             }],
 
-            # For now, Windows builds that |use_aura| should also imply using
-            # ash. This rule should be removed for the future when Windows is
-            # using the aura windows without the ash interface.
-            ['use_aura==1 and OS=="win"', {
-              'use_ash%': 1,
-            }],
-            ['use_ash==1', {
-              'use_aura%': 1,
+            # Ash needs Aura.
+            ['use_aura==0', {
+              'use_ash%': 0,
             }],
 
             # Whether we're a traditional desktop unix.
             ['(OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris") and chromeos==0', {
               'desktop_linux%': 1,
             }, {
               'desktop_linux%': 0,
             }],
 
             # Compute the architecture that we're building on.
@@ skipping 4646 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }