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>
1 file changed