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Mojo serialization should not be blocking, but this CHECK included a
blocking call to Fstat. This prevents DisallowScopedBlocking consumers
from using this type. We now call fstat directly and while this
could block in practice this should be quick for already opened
physical files.

The remaining CHECK calls fcntl with F_GETFL which essentially just
copies an int from the kernel back to the process, so can be
considered non-blocking, and so left in place.

Bug: 1130762
Change-Id: Iced551675d5e8c74ac62b8bcbffc240dbde1aa95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466203Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817104}