Added a comment that File::IsValid can be called outside of file thread.

File::IsValid ends up calling either ScopedFS::is_valid() or
win::ScopedHandle::is_valid() - both of these classes inherit is_valid()
from ScopedGeneric, which implements is_valid() as a memory-only
operation that don't touch the file system:

  bool is_valid() const { return data_.generic != traits_type::InvalidValue(); }

BUG=

Review URL: https://codereview.chromium.org/1409323005

Cr-Commit-Position: refs/heads/master@{#355402}

base/files/file.h

@@ -182,6 +182,9 @@ class BASE_EXPORT File {
   // Creates or opens the given file.
   void Initialize(const FilePath& path, uint32 flags);
 
+  // Returns |true| if the handle / fd wrapped by this object is valid.  This
+  // method doesn't interact with the file system (and is safe to be called from
+  // ThreadRestrictions::SetIOAllowed(false) threads).
   bool IsValid() const;
 
   // Returns true if a new file was created (or an old one truncated to zero