[LayoutNG] Change FindLayoutOpportunity to not create a Vector each time.

This lambda-izes AllLayoutOpportunities, so that FindLayoutOpportunity
doesn't need to iterate over a Vector to find something that fits.

Bug: 635619
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I0cf9d5ce16608032850e326173a9fb4d501a9181
Reviewed-on: https://chromium-review.googlesource.com/c/1193942
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604120}
2 files changed