Fix FilesystemUtils.SourceDirForPath after r306368

TBR=brettw@chromium.org
BUG=348474

Review URL: https://codereview.chromium.org/775543003

Cr-Original-Commit-Position: refs/heads/master@{#306471}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9a760d92ae2316c12009938fa88021a6be726bb8