Add target_arch config variable to .isolate files.

chrome/angle.isolate

 # Copyright 2014 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.
 {
   'includes': [
     '../base/base.isolate',
   ],
   'conditions': [
     ['OS=="win"', {
       'variables': {
         'isolate_dependency_tracked': [
+          '<(PRODUCT_DIR)/d3dcompiler_46.dll',
           '<(PRODUCT_DIR)/libEGL.dll',
           '<(PRODUCT_DIR)/libGLESv2.dll',
-          '<(PRODUCT_DIR)/d3dcompiler_46.dll',
-          # TODO(kbr): really need D3DCompiler_43.dll as well for XP,
-          # but ANGLE doesn't contain the rule which extracts this.
         ],
       },
     }],
+    ['OS=="win" and target_arch=="ia32"', {
+      'variables': {
+        'isolate_dependency_tracked': [
+          # TODO(kbr): really need D3DCompiler_43.dll as well for XP,
+          # but ANGLE doesn't contain the rule which extracts this.
+          #'<(PRODUCT_DIR)/D3DCompiler_43.dll',
+        ],
+      },
+    }],
   ],
 }