Is it specifically Windows 7 only? Not Windows 8 or 10?

These tests are very weird, and it wouldn't surprise me if they left the operating system a little dazed and confused, when run together like that.

What I find weird is that we only see this problem now that PA is in base. The tests themselves have not changed; maybe people were not running them when they were in Blink?

I don't know. I'm on Windows 7. It's possible that it's reproducible also on other Windows versions but I haven't tested this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/57cc1d8512be5a3719e1f5acbe67a3556ee9c2a7

commit 57cc1d8512be5a3719e1f5acbe67a3556ee9c2a7
Author: sigbjornf <sigbjornf@opera.com>
Date: Sun Feb 12 14:32:50 2017

Fix partition_alloc unit tests.

R=haraken
BUG= 691197 ,  684513 

Review-Url: https://codereview.chromium.org/2689103002
Cr-Commit-Position: refs/heads/master@{#449892}

[modify] https://crrev.com/57cc1d8512be5a3719e1f5acbe67a3556ee9c2a7/base/allocator/partition_allocator/PartitionAlloc.md
[modify] https://crrev.com/57cc1d8512be5a3719e1f5acbe67a3556ee9c2a7/base/allocator/partition_allocator/partition_alloc_unittest.cc

Re #1: Was Blink perhaps running these tests in sequence rather than sharded & in parallel?

They should be on google bots ( https://bugs.chromium.org/p/chromium/issues/detail?id=524758#c22 ), but no sharding on opera's side (where this report was from.)

Re #5: Not sure what you mean by "no sharding on opera's side". Do you mean you explicitly run base_unittests with sharding disabled?