GN: Use path service to find executable path

This uses the path service to find the current process' path, removing the platform-specific code nad making it work on Mac.

R=bbudge@chromium.org

Review URL: https://codereview.chromium.org/128693003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243638 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243638