Add gyp support and isolates for default test suites.
Add two default isolates, one (default) for using the
test suite collection we call "default" on the bots. One
(developer_default) for also supporting the way developers
call the driver (i.e. without argument, which includes
the unittests).

BUG=chromium:535160
LOG=n

Review URL: https://codereview.chromium.org/1380593002

Cr-Commit-Position: refs/heads/master@{#31081}