(This is a merge of http://go/wvgerrit/13922 from the Widevine CDM
repository.)
I'm not sure why we chose to pass char* instead of std::string to the
helper functions in DeviceFiles, but it seems to require a lot of
gymnastics of the calling code for minimal gain.
Change-Id: Ie0cdec80ab77c94370648dd74249124aed6e8be1