[gn] Add gn support to gcmole

tools/gcmole/run-gcmole.isolate

 # Copyright 2016 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.
 {
   'variables': {
     'command': [
       'run-gcmole.py',
     ],
     'files': [
       'gccause.lua',
       'gcmole.lua',
       'parallel.py',
       'run-gcmole.py',
       # The following contains all relevant source and gyp files.
+      '../../BUILD.gn',
       '../../base/',
       '../../include/',
       '../../src/',
       '../../test/cctest/',
       '../../testing/gtest/include/gtest/gtest_prod.h',
       '../../third_party/icu/source/',
     ],
   },
   'conditions': [
     ['gcmole==1', {
       'variables': {
         'files': [
           # This assumes gcmole tools have been fetched by a hook
           # into v8/tools/gcmole/gcmole_tools.
           'gcmole-tools/',
         ],
       },
     }],
   ],
 }