Use paths relative to build dir in custom actions.

Also inserts a space between the rule name and the inputs in the generated ninja.

BUG=309227

Review URL: https://codereview.chromium.org/30253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231304 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231304