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

base/files/file_tracing.h

Index: base/files/file_tracing.h
diff --git a/base/files/file_tracing.h b/base/files/file_tracing.h
index 149bd78a9b4a5000543ff3d36cd88397b4bb9ede..92324c9475abe0646d834c21a0eabc80b87eef42 100644
--- a/base/files/file_tracing.h
+++ b/base/files/file_tracing.h
@@ -13,7 +13,7 @@
 
 #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)
@@ -25,6 +25,9 @@ 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.
@@ -61,9 +64,6 @@ class BASE_EXPORT FileTracing {
     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