This adds 5% testing of 'ignition' vs 'liftoff', which tests Turbofan vs
Liftoff for wasm code, and tests Ignition vs Turbofan for javascript
code.
It also adds 3% testing of 'liftoff' (x64) vs 'liftoff' (ia32), which
does standard x64 vs ia32 testing for javascript code.

R=machenbach@chromium.org

Bug: chromium:824098, v8:6600
Change-Id: I6a6afae0300efc33f3535541a11695a7bb32dcc5
Reviewed-on: https://chromium-review.googlesource.com/973161
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52111}