Do not depend on wvcdm::Clock in OEC tests

Merge from widevine repo of http://go/wvgerrit/14669

This fixes two timing-related test failures when running the OEMCrypto
tests in the CE test suite.  The failures were caused by the TestHost
Clock implementation, which is a fake.  Since there is no clear reason
for OEMCrypto/mock to rely on wvcdm::Clock, this replaces it with
time(NULL).  Incidentally, this also makes the time source consistent
with the tests themselves, which were already using time(NULL).

Change-Id: I0fad51f14d45f99526146da05b757d4ba7b6aba0

libwvdrmengine/oemcrypto/mock/src/oemcrypto_usage_table_mock.h

@@ -10,7 +10,6 @@
 #include <string>
 #include <vector>
 
-#include "clock.h"
 #include "lock.h"
 #include "OEMCryptoCENC.h"
 #include "openssl/sha.h"
@@ -73,7 +72,6 @@ class UsageTableEntry {
   std::vector<uint8_t> mac_key_client_;
 
   SessionContext *session_;
-  static wvcdm::Clock clock_;
 };
 
 class UsageTable {