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

base/files/file.h

Index: base/files/file.h
diff --git a/base/files/file.h b/base/files/file.h
index 94ef56fbbc8fae9be21f93a38dbad1e49a06de20..b592c79cc13f2d3442f89fceaadd79d21c45d796 100644
--- a/base/files/file.h
+++ b/base/files/file.h
@@ -357,7 +357,7 @@ class BASE_EXPORT File {
 
   // Creates or opens the given file. Only called if |path_| has no
   // traversal ('..') components.
-  void DoInitialize(uint32 flags);
+  void DoInitialize(uint32 flags, const FilePath& path);

[rvargas (doing something else), 2015/07/02 02:11:05]

tiny nit: Do you mind going back to having path as the first argument?. I also
don't follow why are we documenting on the header that a private member is not
called when the path used is invalid. Granted, that is not part of this CL.

[Dan Beam, 2015/07/02 02:18:06]

Done.

 
   // TODO(tnagel): Reintegrate into Flush() once histogram isn't needed anymore,
   // cf. issue 473337.
@@ -371,8 +371,9 @@ class BASE_EXPORT File {
   MemoryCheckingScopedFD file_;
 #endif
 
-  // Path that |Initialize()| was called with. Only set if safe (i.e. no '..').
-  FilePath path_;
+  // Path that |Initialize()| was called with. Only set if safe (i.e. no '..')

[rvargas (doing something else), 2015/07/02 02:11:06]

I think the safe reference is stale at this point, no?. If the path is not safe,
Initialization will fail and IsValid() will return false, which means the object
is not usable at that point (do we really want to document something about a
member that is only used for tracing, with objects on that state?)

[Dan Beam, 2015/07/02 02:18:06]

Done.

+  // and if file tracing is enabled when |Initialize()| is called.
+  FilePath tracing_path_;
 
   // Object tied to the lifetime of |this| that enables/disables tracing.
   FileTracing::ScopedEnabler trace_enabler_;