Use google::GLOG_WARNING instead of WARNING in tests to support MSVC.

- GLOG_NO_ABBREVIATED_SEVERITIES is the default on Windows, in which
  case google::WARNING is not defined.
- Remove Ceres-specific redefinition of WARNING in mock-log.h fork
  in place of using non-abbreviated severity in the one place in
  levenberg_marquardt_strategy_test where ScopedMockLog is actually
  used.
- Remove unnecessary dependency of gradient_checking_cost_function_test
  on ScopedMockLog.

Change-Id: I9fb540f638037b6015fd264cfc618c9d60f5686c

internal/ceres/gradient_checking_cost_function_test.cc

@@ -45,7 +45,6 @@
 #include "ceres/types.h"
 #include "glog/logging.h"
 #include "gmock/gmock.h"
-#include "gmock/mock-log.h"
 #include "gtest/gtest.h"
 
 namespace ceres {
@@ -55,7 +54,6 @@ using std::vector;
 using testing::AllOf;
 using testing::AnyNumber;
 using testing::HasSubstr;
-using testing::ScopedMockLog;
 using testing::_;
 
 // Pick a (non-quadratic) function whose derivative are easy: