Since --trace-ignition now has to be enabled at snapshot-building time,
this patch adds it as a gn build option.

Change-Id: I5d55339a7be7eef4e1f9da46ec44fbfd431325b7
Reviewed-on: https://chromium-review.googlesource.com/474905Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44583}