Add a flag to explicitly filter scripts in ignition and use it for the test262
variant. The previous approach of overloading ignition-filter meant that only
top-level code was getting compiled through ignition.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1396493002

Cr-Commit-Position: refs/heads/master@{#31164}