*Scan: Make sure to properly construct/destroy root maps

std::unordered_map::clear() isn't guaranteed to destroy the backing.
This caused a problem when a subsequently running test accesses the old
backing store.

The CL makes sure to explicitly call the destructor in
ReinitForTesting() and construct PCScanInternal with placement new to
fix the issue.

Bug: 11297512
Change-Id: I476b0a252ac0899a640158cae3446cbe143d5648
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3010121


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#899193}