vpn-manager: use MOCK_CONST_METHOD for const func

BUG=chromium:233350
TEST=FEATURES="test" CXXFLAGS="-clang" emerge-lumpy vpn-manager
     passes.

Change-Id: I29f04071e0c95a3385a9026bd93c4aa37a28557e
Reviewed-on: https://gerrit.chromium.org/gerrit/48566
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
diff --git a/daemon_mock.h b/daemon_mock.h
index 82354b8..286e1ac 100644
--- a/daemon_mock.h
+++ b/daemon_mock.h
@@ -21,7 +21,7 @@
   MOCK_METHOD0(FindProcess, bool());
   MOCK_METHOD0(IsRunning, bool());
   MOCK_METHOD0(Terminate, bool());
-  MOCK_METHOD0(GetPid, pid_t());
+  MOCK_CONST_METHOD0(GetPid, pid_t());
 };
 
 }  // namespace vpn_manager