Mojo: Add GetMinimumLogLevel() to the logger interface.

mojo/public/c/environment/logger.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_C_ENVIRONMENT_LOGGER_H_
 #define MOJO_PUBLIC_C_ENVIRONMENT_LOGGER_H_
 
 #include <stdint.h>
 
 // |MojoLogLevel|: Used to specify the type of log message. Values are ordered
 // by severity (i.e., higher numerical values are more severe).
 
 typedef int32_t MojoLogLevel;
 
 #ifdef __cplusplus
 const MojoLogLevel MOJO_LOG_LEVEL_VERBOSE = -1;
 const MojoLogLevel MOJO_LOG_LEVEL_INFO = 0;
 const MojoLogLevel MOJO_LOG_LEVEL_WARNING = 1;
 const MojoLogLevel MOJO_LOG_LEVEL_ERROR = 2;
 const MojoLogLevel MOJO_LOG_LEVEL_FATAL = 3;
 #else
 #define MOJO_LOG_LEVEL_INFO ((MojoLogLevel) 0)
 #define MOJO_LOG_LEVEL_WARNING ((MojoLogLevel) 1)
 #define MOJO_LOG_LEVEL_ERROR ((MojoLogLevel) 2)
 #define MOJO_LOG_LEVEL_FATAL ((MojoLogLevel) 3)
 #endif
 
+// Structure with basic logging functions (on top of which more friendly logging
+// macros may be built). The functions are thread-safe.
 struct MojoLogger {
+  // Logs |message| at level |log_level| if |log_level| is at least the current
+  // minimum log level. If |log_level| is |MOJO_LOG_LEVEL_FATAL| (or greater),
+  // aborts the application/process.
   void (*LogMessage)(MojoLogLevel log_level, const char* message);
+
+  // Gets the minimum log level (see above), which will always be at most
+  // |MOJO_LOG_LEVEL_FATAL|. (Though |LogMessage()| will automatically avoid
+  // logging messages below the minimum log level, this may be used to avoid
+  // extra work.)
+  MojoLogLevel (*GetMinimumLogLevel)(void);
 };
 
 #endif  // MOJO_PUBLIC_C_ENVIRONMENT_LOGGER_H_