GitLab

The sets and maps are typically small or empty and created once. flat_set and
flat_map will have better performance for these workloads.

See the bug for histograms of sizes.

Reland of https://codereview.chromium.org/2821353002 with fix.
Reland of https://codereview.chromium.org/2823043002 with fix.

BUG=712295
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel
TBR=jbauman@chromium.org

Review-Url: https://codereview.chromium.org/2831663002
Cr-Commit-Position: refs/heads/master@{#465681}