mock_platform: fix argument count for GetGroupId mock

It said 3 args but it really only needs 2.

TEST=unittests compile and pass. They didn't before
BUG=none

Change-Id: I68d7de591aa24159669bf3d8ee5d49e787595424

R=gauravsh@chromium.org,davidjames@chromium.org

Review URL: http://codereview.chromium.org/6771026
diff --git a/mock_platform.h b/mock_platform.h
index 31ee451..aaa6c8a 100644
--- a/mock_platform.h
+++ b/mock_platform.h
@@ -38,7 +38,7 @@
   MOCK_METHOD3(SetOwnership, bool(const std::string&, uid_t, gid_t));
   MOCK_METHOD3(SetOwnershipRecursive, bool(const std::string&, uid_t, gid_t));
   MOCK_METHOD3(GetUserId, bool(const std::string&, uid_t*, gid_t*));
-  MOCK_METHOD3(GetGroupId, bool(const std::string&, gid_t*));
+  MOCK_METHOD2(GetGroupId, bool(const std::string&, gid_t*));
   MOCK_CONST_METHOD1(AmountOfFreeDiskSpace, int64(const std::string&));
   MOCK_METHOD2(Symlink, bool(const std::string&, const std::string&));
   MOCK_METHOD4(Exec, bool(const std::string&, const std::vector<std::string>&,