Reduce memory consumption on LongTaskObserver::DidProcessTask

https://crrev.com/c/2909934 fixed a security issue, but it introduced a
copy operation for each DidProcessTask for a long task. We see a memory
regression on the change, and this is an attempt to mitigate the
regression.

Bug: 1210487, 1211539
Change-Id: Ib9101e29d70fadb11b7967754e847bb5cc754feb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2915153


Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#886221}