Pull GN @ 252040, update calls

Updates all rebase_path calls to use the new parameter ordering.

Use the new getenv function and delete the Python script we had to get the home directory.

Takes advantage of the new ability to have the default value of declare_args blocks be dependent on other values. This simplifies some code.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/164773005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252065 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252065