This adds a method to convert a string containing a list of
gn --args -like assignments to a python dict.

(ToGNString() already supports the opposite direction).

R=brettw@chromium.org, hashimoto@chromium.org
BUG=561142

Review URL: https://codereview.chromium.org/1847333005

Cr-Commit-Position: refs/heads/master@{#385336}