This executes an action as part of the build, writing a json
configuration that includes all build flags relevant to v8
testing.

The test runner will derive all build-dependent flags from
the file if it detects it.

BUG=chromium:474921

Review-Url: https://codereview.chromium.org/2106423002
Cr-Commit-Position: refs/heads/master@{#37446}