net: build fix about CertificateList
net: build fix about CertificateList

clang complains it doesn't initialize const member, so this CL calls the
default constructor explicitly.

../../net/cert/multi_threaded_cert_verifier.cc:430:25: error: default initialization of an object of const type 'const CertificateList' (aka 'const vector<scoped_refptr<net::X509Certificate> >') without a user-provided default constructor
  const CertificateList empty_cert_list;
                        ^
1 error generated.

Review URL: https://codereview.chromium.org/1572253002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368585}

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