Added support for relative paths for the .gclient cache_dir field.
Added support for relative paths for the .gclient cache_dir field.

R=iannucci@chromium.org
BUG=None
TEST=gclient sync with a relative and absolute "cache_dir"

Review URL: https://codereview.chromium.org/415713002

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@288146 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools