[base] Make InsecureRandomGenerator seeded in its constructor.

This changes base::InsecureRandomGenerator so that there is no need to
seed it before using, as its constructor now makes sure it is seeded.
Since the constructor is no longer constexpr, its use in PartitionAlloc
is updated to initialize it upon first use.

Change-Id: Iae3c63102ef5bf0416751956874e8ffcd045685b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3006803


Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#914692}