libvpx: Fix generated offsets to work with WebView

For WebView use hardcoded unpack lib as an input trigger, but then
an abspath to find the lib.

BUG=377062
TBR=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/325313007

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libvpx@277778 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
1 file changed