Clarify Net.QuicSession.ConnectionCloseErrorCode* histogram summaries.

Replace "Google QUIC" with "QUIC" in some places that were not specific
to Google QUIC.

Replace "name ending in .google.com" with "a Google server", since there
are multiple hostnames used (see net::IsGoogleHost function).

Change-Id: Ic70f6f7813f83d15e6641b5a0cd88c2bd786e526
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2659556


Reviewed-by: David Schinazi <dschinazi@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848707}