Added a comment that File::IsValid can be called outside of file thread.
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355402}

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