Avoiding malloc and unique_ptr seems to have positive effects on a
representative facebook and cnn workload.
This makes arrow functions and eager functions potentially more
expensive since the DataGatheringScope is now always stack-allocated
and only the full initialization happens conditionally.

Change-Id: Ibf1c1308a7db464f7c5d2bafd61560e4cabf0ce9
Reviewed-on: https://chromium-review.googlesource.com/c/1382733Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58340}