Mojo: Make GetDefault{AsyncWaiter,Logger} static methods of Environment.

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_VERBOSE ((MojoLogLevel) -1)
 #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.
+// macros may be built). The functions are thread-safe, except for
+// |SetMinimumLogLevel()| (see below).
 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);
+
+  // Sets the minimum log level (see above) to the lesser of |minimum_log_level|
+  // and |MOJO_LOG_LEVEL_FATAL|.
+  //
+  // Warning: This function may not be thread-safe, and should not be called
+  // concurrently with other |MojoLogger| functions. (In some environments --
+  // such as Chromium -- that share a logger across applications, this may mean
+  // that it is almost never safe to call this.)
+  void (*SetMinimumLogLevel)(MojoLogLevel minimum_log_level);
 };
 
 #endif  // MOJO_PUBLIC_C_ENVIRONMENT_LOGGER_H_