Only set File::[tracing_]path_ when file tracing is enabled.

base/files/file_tracing.h

 // Copyright 2015 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 BASE_FILES_FILE_TRACING_H_
 #define BASE_FILES_FILE_TRACING_H_
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/macros.h"
 
 #define FILE_TRACING_PREFIX "File"
 
 #define SCOPED_FILE_TRACE_WITH_SIZE(name, size) \
     FileTracing::ScopedTrace scoped_file_trace; \
-    if (scoped_file_trace.ShouldInitialize()) \
+    if (FileTracing::IsCategoryEnabled()) \
       scoped_file_trace.Initialize(FILE_TRACING_PREFIX "::" name, this, size)
 
 #define SCOPED_FILE_TRACE(name) SCOPED_FILE_TRACE_WITH_SIZE(name, 0)
 
 namespace base {
 
 class File;
 class FilePath;
 
 class BASE_EXPORT FileTracing {
  public:
+  // Whether the file tracing category is enabled.
+  static bool IsCategoryEnabled();
+
   class Provider {
    public:
     // Whether the file tracing category is currently enabled.
     virtual bool FileTracingCategoryIsEnabled() const = 0;
 
     // Enables file tracing for |id|. Must be called before recording events.
     virtual void FileTracingEnable(void* id) = 0;
 
     // Disables file tracing for |id|.
     virtual void FileTracingDisable(void* id) = 0;
@@ skipping 16 matching lines @@
    public:
     ScopedEnabler();
     ~ScopedEnabler();
   };
 
   class ScopedTrace {
    public:
     ScopedTrace();
     ~ScopedTrace();
 
-    // Whether this trace should be initialized or not.
-    bool ShouldInitialize() const;
-
     // Called only if the tracing category is enabled. |name| is the name of the
     // event to trace (e.g. "Read", "Write") and must have an application
     // lifetime (e.g. static or literal). |file| is the file being traced; must
     // outlive this class. |size| is the size (in bytes) of this event.
     void Initialize(const char* name, File* file, int64 size);
 
    private:
     // The ID of this trace. Based on the |file| passed to |Initialize()|. Must
     // outlive this class.
     void* id_;
 
     // The name of the event to trace (e.g. "Read", "Write"). Prefixed with
     // "File".
     const char* name_;
 
     DISALLOW_COPY_AND_ASSIGN(ScopedTrace);
   };
 
   DISALLOW_COPY_AND_ASSIGN(FileTracing);
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_FILE_TRACING_H_