public static byte[] decodeWebSafe(byte[] source, int off, int len)
      throws Base64DecoderException {
    return decode(source, off, len, WEBSAFE_DECODABET);
  }
Decodes web safe Base64 content in byte array format and returns
the decoded byte array.
Web safe encoding uses '-' instead of '+', '_' instead of '/'

@param source The Base64 encoded data
@param off    The offset of where to begin decoding
@param len    The length of characters to decode
@return decoded data
android/android_framework/base/src/main/java/com/google/android/vending/licensing/util/Base64.java