libvpx: Change generated offsets to work with Webview

Webview cannot hardcode the output directory for libvpx libs.

BUG=377062
TBR=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/320923003

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libvpx@275816 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
1 file changed