Add missing (and remove superfluous) 'explicit' from constructors.

base/debug/stack_trace_posix.cc

 // Copyright (c) 2012 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.
 
 #include "base/debug/stack_trace.h"
 
 #include <errno.h>
 #include <execinfo.h>
 #include <fcntl.h>
 #include <signal.h>
@@ skipping 230 matching lines @@
     // stack dumping signal handler). NO malloc or stdio is allowed here.
     ignore_result(HANDLE_EINTR(write(STDERR_FILENO, output, strlen(output))));
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PrintBacktraceOutputHandler);
 };
 
 class StreamBacktraceOutputHandler : public BacktraceOutputHandler {
  public:
-  StreamBacktraceOutputHandler(std::ostream* os) : os_(os) {
+  explicit StreamBacktraceOutputHandler(std::ostream* os) : os_(os) {
   }
 
   virtual void HandleOutput(const char* output) {
     (*os_) << output;
   }
 
  private:
   std::ostream* os_;
 
   DISALLOW_COPY_AND_ASSIGN(StreamBacktraceOutputHandler);
@@ skipping 139 matching lines @@
     *ptr = *start;
     *start++ = ch;
   }
   return buf;
 }
 
 }  // namespace internal
 
 }  // namespace debug
 }  // namespace base