Fix warning in Xcode about only building the active arch in Debug.

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 4626 matching lines @@
       'make_global_settings': [
         ['CC', '<(sysroot)/../bin/mipsel-linux-gnu-gcc'],
         ['CXX', '<(sysroot)/../bin/mipsel-linux-gnu-g++'],
         ['LINK', '$(CXX)'],
         ['CC.host', '<!(which gcc)'],
         ['CXX.host', '<!(which g++)'],
         ['LINK.host', '<!(which g++)'],
       ],
     }],
   ],
+  'configurations': {
+    # DON'T ADD ANYTHING NEW TO THIS BLOCK UNLESS YOU REALLY REALLY NEED IT!
+    # This block adds *project-wide* configuration settings to each project
+    # file.  It's almost always wrong to put things here.  Specify your
+    # custom |configurations| in target_defaults to add them to targets instead.
+    'Debug': {
+      'xcode_settings': {
+        # Enable 'Build Active Architecture Only' for Debug. This
+        # avoids a project-level warning in Xcode.
+        'ONLY_ACTIVE_ARCH': 'YES',
+      },
+    },
+  },
   'xcode_settings': {
     # DON'T ADD ANYTHING NEW TO THIS BLOCK UNLESS YOU REALLY REALLY NEED IT!
     # This block adds *project-wide* configuration settings to each project
     # file.  It's almost always wrong to put things here.  Specify your
     # custom xcode_settings in target_defaults to add them to targets instead.
 
     'conditions': [
       # In an Xcode Project Info window, the "Base SDK for All Configurations"
       # setting sets the SDK on a project-wide basis. In order to get the
       # configured SDK to show properly in the Xcode UI, SDKROOT must be set
@@ skipping 43 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',
   },
 }