gn assert that DCHECKs are enabled when expected

This adds build-time asserts that we do have DCHECKs enabled when either
of dcheck_is_configurable or enable_expensive_dchecks is turned on as
those flags would be unexpected no-ops if DCHECKs are off overall.

These asserts would've caught that the "win-asan" bot inadvertently got
its configuration updated to:

  dcheck_always_on = false
  dcheck_is_configurable = true

Which made the latter a no-op and disabled DCHECKs for the build
entirely even though it expected configurable DCHECKs.

Bug: 1328038
Change-Id: Ifd3fa6f7179caf9541cb52a5ef88eb1bb1a7b25c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3866495
Commit-Queue: Peter Boström <pbos@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Auto-Submit: Peter Boström <pbos@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1041271}