Pull GN @ 252040, update calls

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

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=252065

[brettw, 2014-02-14 23:30:41]

This will incorporate a GN binary push once
https://codereview.chromium.org/161783002/ is landed.

[scottmg, 2014-02-14 23:55:35]

lgtm

[brettw, 2014-02-19 20:32:12]

Committed patchset #5 manually as r252065 (presubmit successful).