Cross compiling: don't rewrite dirs if they're already prefixed.
Cross compiling: don't rewrite dirs if they're already prefixed.

When compiling with the new portage build system for Chromium OS,
paths passed in for includes, libraries, etc., are already properly
prefixed with the sysroot and do not need to be again prefixed with
the sysroot.

Review URL: http://codereview.chromium.org/652067

git-svn-id: http://src.chromium.org/svn/trunk/src@39655 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 11d4120747458b4b534bfc0e256402768512c3b4
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser