net: Add histogram for H2/H3 session source

The `Net.NetworkTransaction.SessionSource.{Protocol}` histogram tracks
whether a request was served by a newly created session or an
existing one. This data will help analyze session reuse patterns.

Also fix a bug in HTTP/1.1 socket reuse cases. Previously these were
treated as using existing sessions but it should not be treated as
such.

NO_IFTTT=Not sure how to fix warning

Bug: 441134585
Change-Id: I0b28093313d61dab059aba12ed0c72a0e21bb374
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6938435
Reviewed-by: Nidhi Jaju <nidhijaju@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519822}