GN: Add the ability to trim whitespace from script results.

Adds an optional "trim" mode that can trim whitespace from the beginnings and endings of script results when converting them to GN values.

This also adds default arguments for exec_script to simplify calling in cases where the arguments are not needed.

R=bbudge@chromium.org

Review URL: https://codereview.chromium.org/129043005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244586 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244586