PCScan: Wrap calls inside DCHECKS in hot pcscan paths in DCHECK_IS_ON()

It may happen that the compiler fails to optimize away a certain call.
Make sure it doesn't happen with DCHECK_IS_ON().

Bug: 11297512
Change-Id: I26f93c785e62655310d450c712ab2294d98f2b17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2679481


Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#851455}