Set test_isolation_outdir to '' by default

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 386 matching lines @@
       # Whether tests targets should be run, archived or just have the
       # dependencies verified. All the tests targets have the '_run' suffix,
       # e.g. base_unittests_run runs the target base_unittests. The test target
       # always calls tools/swarm_client/isolate.py. See the script's --help for
       # more information and the valid --mode values. Meant to be overriden with
       # GYP_DEFINES.
       # TODO(maruel): Converted the default from 'check' to 'noop' so work can
       # be done while the builders are being reconfigured to check out test data
       # files.
       'test_isolation_mode%': 'noop',
-      # It must not be '<(PRODUCT_DIR)' alone, the '/' is necessary otherwise
-      # gyp will remove duplicate flags, causing isolate.py to be confused.
-      'test_isolation_outdir%': '<(PRODUCT_DIR)/isolate',
+      # If no directory is specified then a temporary directory will be used.
+      'test_isolation_outdir%': '',
 
       'sas_dll_path%': '<(DEPTH)/third_party/platformsdk_win7/files/redist/x86',
       'wix_path%': '<(DEPTH)/third_party/wix',
 
       'conditions': [
         # TODO(epoger): Figure out how to set use_skia=1 for Mac outside of
         # the 'conditions' clause.  Initial attempts resulted in chromium and
         # webkit disagreeing on its setting.
         ['OS=="mac"', {
           'use_skia%': 1,
@@ skipping 3565 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',
   },
 }