[REG] Fixed Build failure when sometimes command output contain SOFT HYPHEN(0xad)

There are 2 changes on RunCommand() function:
1 In verbose mode, remove .decode() to make sure the print info is original
  as getting from PIPE.
2 Change the return value to ".decode([encoding], [errors='replace'])",
  make sure it build passed when the command output contain 0xad.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2468