DrCov: update genhtml to handle Windows path in cygwin perl
DrCov: update genhtml to handle Windows path in cygwin perl

- convert \ to / in $filename to support proccessing Windows cov files
  using cygwin perl

Review-URL: https://codereview.appspot.com/274530043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio