MB: Add --isolate-map-file flag for overriding default isolate map.

tools/mb/mb_unittest.py

Index: tools/mb/mb_unittest.py
diff --git a/tools/mb/mb_unittest.py b/tools/mb/mb_unittest.py
index b0957008817b4f1ceb08f0532d9c726633fcdb5a..602b98dadeac5c5b8bdf6fed5880e76c061a9a6f 100755
--- a/tools/mb/mb_unittest.py
+++ b/tools/mb/mb_unittest.py
@@ -22,12 +22,15 @@ class FakeMBW(mb.MetaBuildWrapper):
     if win32:
       self.chromium_src_dir = 'c:\\fake_src'
       self.default_config = 'c:\\fake_src\\tools\\mb\\mb_config.pyl'
+      self.default_isolate_map = ('c:\\fake_src\\testing\\buildbot\\'
+                                  'gn_isolate_map.pyl')
       self.platform = 'win32'
       self.executable = 'c:\\python\\python.exe'
       self.sep = '\\'
     else:
       self.chromium_src_dir = '/fake_src'
       self.default_config = '/fake_src/tools/mb/mb_config.pyl'
+      self.default_isolate_map = '/fake_src/testing/buildbot/gn_isolate_map.pyl'
       self.executable = '/usr/bin/python'
       self.platform = 'linux2'
       self.sep = '/'