This makes the isolate-driver script rebase all paths to be
relative to the isolate-file location. This is an assumption
of the go binaries and is needed for batcharchive to work.

In gyp, actions were executed relative to the gyp file that
specified them, while in gn it's relative to the product dir.

BUG=chromium:474921

Review-Url: https://codereview.chromium.org/2039873002
Cr-Commit-Position: refs/heads/master@{#36746}