[Lorenz] Adapt graph generation for older checkouts

In older checkouts, the .javac.jar is in gen/ rather than obj/

Change-Id: I1a149fd02e463b1fabd2be87e584bb0f9c759c19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518700


Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824907}