This is a simplified version of chromium's isolate driver:
- 'prepare' will create a *.isolate.gen.json with the
arguments to be used later.
- other modes will just be passed through to the isolate
script.
BUG=chromium:535160
LOG=n
NOTRY=true
Review URL: https://codereview.chromium.org/1378133005
Cr-Commit-Position: refs/heads/master@{#31046}
