iwa: Add UMA metric measuring Signed Web Bundle file lengths

This metric measures the file lengths of Signed Web Bundles used for
Isolated Web Apps in MiB. It will allow us to gather insights into the
bundle sizes used in the wild.

Expiry is set to one year from now to allow us to monitor this metric
over the course of launching Isolated Web Apps next year.

Low-Coverage-Reason: base::File is hard to test / mock
Bug: 1382396
Change-Id: I36ec7677831c2afb13fabf3ca0c17bc5f29e03b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4014960


Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Christian Flach <cmfcmf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076742}