PCScan: Avoid OOB checks in std::bitset

This CL replaces std::bitset with a simple bitmap implementation that
avoids redundant OOB checks in the pcscan loop.

Bug: 11297512
Change-Id: Ided593f738cef4984363b35554b55dab7060579d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2658178


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848826}