Add checked casts for the length of a base::StringPiece.

Since not all consumers of base::StringPiece use a full warning
configuration, this prevents accidentally passing a signed value
into the length, which could result in a buffer overflow happening.

A test is added to verify this new behavior.

A few small companion changes were needed to get green bots:

1. websocket_frame.h was using the 'enum hack' for constants, these
were changed to real constants, otherwise base::CheckedNumeric
would fail with static assert in tests as an enum is not arithmetic.
2. CheckedNumeric constructor could incorrectly match non-numeric
types. This caused an incorrect overloaded function match in the
suggestion parser unittest [1]. The CheckedNumeric constructor was
therefore disabled for non-arithmetic types.

[1] - https://ci.chromium.org/ui/p/chromium/builders/try/linux-rel/1201914/overview

BUG=1385166

Change-Id: Ic01aec5e08fc4121373f5429593a340a0497c7d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4021423
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1073064}