media_source_dshow: Use string_util_win.
media_source_dshow: Use string_util_win.

Remove locally defined functions.

Change-Id: I31d57d937e8fb0bd40d6197cec4d6dabe0e04b86

felipebetancur/webmlive,webmproject/webmlive,webmproject/webmlive,felipebetancur/webmlive,felipebetancur/webmlive,webmproject/webmlive,webmproject/webmlive,felipebetancur/webmlive,webmproject/webmlive,felipebetancur/webmlive