[swarming] Isolate custom libcxx.

src/base.isolate

 # Copyright 2015 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'conditions': [
+    ['use_custom_libcxx==1', {
+      'variables': {
+        'files': [
+          '<(PRODUCT_DIR)/lib/libc++.so',
+        ],
+      },
+    }],
     ['v8_use_snapshot=="true" and v8_use_external_startup_data==1', {
       'variables': {
         'files': [
           '<(PRODUCT_DIR)/natives_blob.bin',
           '<(PRODUCT_DIR)/snapshot_blob.bin',
         ],
       },
     }],
     ['OS=="linux" and component=="shared_library" and target_arch=="ia32"', {
       'variables': {
         'files': [
           '<(PRODUCT_DIR)/lib/',
         ],
       },
     }],
   ],
 }