Instead of baking in a relative path and expecting the app to run from
a fixed directory, we can change the CWD at runtime so the layers can
load relative to the current executable directory.

Future alternatives could include modifying the layers SDK to provide
a path dynamically, but for now the relative paths must be baked in
at compile-time.

BUG=angleproject:1319
BUG=chromium:677841

Change-Id: I443b6b35d38276ea667cdf08ec2204ea280b6cec
Reviewed-on: https://chromium-review.googlesource.com/425441
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>