Fixes two bugs in analyzer

Wasn't delaing with relative paths outside the repo. NACL does this
for things like /usr/include/...
When combining relative paths I was missing adding a '/'. This result
in paths like baseobserver_list.h instead of base/observer_list.h.

BUG=383609
TEST=covered by unit tests
R=mark@chromium.org

Review URL: https://codereview.chromium.org/383893003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1949 78cadc50-ecff-11dd-a971-7dbc132099af