headless: Optionally support external v8 startup data

Change-Id: I2ec1a06c9874c70c04bc49e0517bb5e4e039d788
Reviewed-on: https://chromium-review.googlesource.com/906750
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535687}
1 file changed