This method returns position of importing stmt in module source.

R=neis@chromium.org

Bug: chromium:721589
Change-Id: I8639796a001fdfec7cf5aa1bf1a27493f7a757a9
Reviewed-on: https://chromium-review.googlesource.com/541322Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46105}