Initialize SCT fields.

This fixes an MSan error introduced by
https://codereview.chromium.org/2294373002/

==31610==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0xa7ce15a in WriteBytesCommon base/pickle.cc:478:3
    #1 0xa7ce15a in WriteBytesStatic<4> base/pickle.cc:443:0
    #2 0x98c5407 in WritePOD<int> base/pickle.h:367:5
    #3 0x98c5407 in WriteInt base/pickle.h:229:0
    #4 0x98c5407 in Persist net/cert/signed_certificate_timestamp.cc:43:0
    #5 0x991c383 in Persist net/http/http_response_info.cc:381:18
    #6 0x2cb75d0 in TestBody net/http/http_response_info_unittest.cc:111:18

  Uninitialized value was stored to memory at
    #0 0x98c57fd in Persist net/cert/signed_certificate_timestamp.cc:43:3
    #1 0x991c383 in Persist net/http/http_response_info.cc:381:18
    #2 0x2cb75d0 in TestBody net/http/http_response_info_unittest.cc:111:18

  Uninitialized value was created by a heap allocation
    #0 0x8da8c2 in operator new(unsigned long) ??:0
    #1 0xa68c2f8 in GetX509CertSCT net/test/ct_test_util.cc:220:14
    #2 0x2cb7451 in TestBody net/http/http_response_info_unittest.cc:104:3

Review-Url: https://codereview.chromium.org/2337983006
Cr-Commit-Position: refs/heads/master@{#418956}

net/cert/signed_certificate_timestamp.cc

@@ -28,7 +28,8 @@ bool SignedCertificateTimestamp::LessThan::operator()(
   return lhs->version < rhs->version;
 }
 
-SignedCertificateTimestamp::SignedCertificateTimestamp() {}
+SignedCertificateTimestamp::SignedCertificateTimestamp()
+    : version(V1), origin(SCT_EMBEDDED) {}
 
 SignedCertificateTimestamp::~SignedCertificateTimestamp() {}
 
@@ -77,7 +78,7 @@ SignedCertificateTimestamp::CreateFromPickle(base::PickleIterator* iter) {
   return sct;
 }
 
-LogEntry::LogEntry() {}
+LogEntry::LogEntry() : type(LOG_ENTRY_TYPE_X509) {}
 
 LogEntry::~LogEntry() {}
 
@@ -87,7 +88,8 @@ void LogEntry::Reset() {
   tbs_certificate.clear();
 }
 
-DigitallySigned::DigitallySigned() {}
+DigitallySigned::DigitallySigned()
+    : hash_algorithm(HASH_ALGO_NONE), signature_algorithm(SIG_ALGO_ANONYMOUS) {}
 
 DigitallySigned::~DigitallySigned() {}