Add MB commands for generating isolates and running them.

Since the logic for handling isolates in GN is split between
the GN and MB tools, it's harder to build and test isolates
with GN than it is with GYP (where you can just say 'ninja base_unittests_run'
to build the isolate).

This patch adds two new commands to the MB tool to make things easier.

The first is `mb isolate`, which is the equivalent of the
GYP _run targets: it creates the .isolate and .isolated.gen.json files.

The second is `mb run`, which is a convenient wrapper command that
runs mb isolate, runs ninja to ensure that the test is actually built
and up-to-date, and then runs the swarming_client commands to actually
run the test locally (in an isolated directory).

Both commands will work either off of an existing build directory or
in a new directory (if given the -m/-b or -c flags).

R=kbr@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1370373005

Cr-Commit-Position: refs/heads/master@{#352217}

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