Mojo: Add a Mojo-specific multiprocess test base class.

mojo/mojo.gyp

Index: mojo/mojo.gyp
diff --git a/mojo/mojo.gyp b/mojo/mojo.gyp
index 015a9d8b0c8f9641e0d9326bbe911b54d4ff306d..7cb5bec64ba33119a7ec559a6a9ac7279b2eb3e8 100644
--- a/mojo/mojo.gyp
+++ b/mojo/mojo.gyp
@@ -40,9 +40,12 @@
       'dependencies': [
         '../base/base.gyp:base',
         '../base/base.gyp:test_support_base',
+        '../testing/gtest.gyp:gtest',
         'mojo_system',
       ],
       'sources': [
+        'common/test/multiprocess_test_base.cc',
+        'common/test/multiprocess_test_base.h',
         'common/test/run_all_unittests.cc',
       ],
     },
@@ -183,6 +186,7 @@
         'common/common_similarity_traits_unittest.cc',
         'common/handle_watcher_unittest.cc',
         'common/message_pump_mojo_unittest.cc',
+        'common/test/multiprocess_test_base_unittest.cc',
       ],
       'conditions': [
         ['OS == "win"', {