Made logging macros evaluate its stream arguments lazily.

base/logging.cc

Index: base/logging.cc
diff --git a/base/logging.cc b/base/logging.cc
index f0c6217cbdd823288855b27bd125e934b90fdd00..ce05d21e8f5c0021279d1c315c32fd666374beb1 100644
--- a/base/logging.cc
+++ b/base/logging.cc
@@ -561,8 +561,9 @@ void LogMessage::Init(const char* file, int line) {
 }
 
 LogMessage::~LogMessage() {
-  // TODO(brettw) modify the macros so that nothing is executed when the log
-  // level is too high.
+  // The macros in logging.h should already avoid creating LogMessages
+  // when this holds, but it's possible that users create LogMessages
+  // directly (e.g., using LOG_STREAM() directly).
   if (severity_ < min_log_level)
     return;