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There are a few places where we access a PartitionRoot with the "wrong"
thread safety template parameter. This is fragile, but also forces us to
remove some DCHECK()s as their layouts don't match. This CL makes sure
the layouts do match, which then removes the issue.

The only downside is making PartitionRoot<NotThreadSafe> a tiny bit
larger, but otherwise this does not add any runtime cost.

Bug: 1092288
Change-Id: I7e24eceacd03682eab7693730a4fc7db973d6f09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368874
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800963}