PCScan: Don't consider decommitted super-pages as containing quarantine

Super pages may have all their slot spans decommitted (or empty). In
such pages there must be no quarantined objects. PCScan can make use of
that and bail out earlier in the lookup procedure.

v8.browsing_desktop cnn:2020 numbers:
pa:partition_alloc:pcscan:scan:  avg:  -2.8% sum:  -3.3%
pa:partition_alloc:pcscan:clear: avg: -17.1% sum: -17.5%
pa:partition_alloc:pcscan:       avg:  -4.2% sum:  -4.7%

Bug: 11297512
Change-Id: I44bb4e3962e19c3201f46e0b1259d2658d136517
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2661457


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849134}