[webdriver] Format pack.sh

- Make http://www.shellcheck.net/ happy
- Follow http://google-styleguide.googlecode.com/svn/trunk/shell.xml
http://www.shellcheck.net/ reports an error "SC2086 Double quote to prevent globbing and word splitting." for this line and lines 53 and 76.

However, `cp -arf "${SRC_ROOT}/*" "${BUILD_ROOT}/"` gets an error in Ubuntu terminal.

Any thoughts?