[PM] Optimize GetAssociatedPageNodes & GetAssociatedProcessNodes

Elements are accumulated in a vector and then transformed into a
flat_set instead of accumulating straight into the flat_set.

Change-Id: Ia921996203ce9c0a129621ddc147ca1dce89c08e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5608155


Reviewed-by: Joe Mason <joenotcharles@google.com>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312597}