Use 'realpath' instead of 'abspath' for self-discovery. This fixes an
issue where 'gsutil' cannot find its dependencies when it is referenced
through a symbolic link.

Specifically, the symlink was: /b/scripts => /b/build/scripts
'gsutil' was called as:
/b/scripts/slave/../../../depot_tools/third_party/gsutil/gsutil

Because '/b/scripts/slave' resolves to '/b/build/scripts/slave', the three
parent directory operators actually resolve to '/b', but 'os.path.abspath'
blindly resolves them to '/'.

BUG=chromium:474442
TEST=live
  - Signed onto failing bot, made this change, 'gsutil' works.

R=hinoka@chromium.org

Review URL: https://codereview.chromium.org/1060353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294712 0039d316-1c4b-4281-b951-d872f2087c98