Makes matching of build files work when absolute

Matching wasn't working if paths supplied were absolute. gyp_chromium
triggers absolute paths, so picking up modified gyp/gypi files wasn't
working when analyzer run from chrome.

BUG=109173
TEST=none
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/534793002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1972 78cadc50-ecff-11dd-a971-7dbc132099af