Return the actual length even when the buffer is neutered (we used
to return 0). This avoids confusion and makes the behavior consistent
with byte_offset() and byte_length().

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I998f12fa4a428f8555f62e1535247f571ab053f2
Reviewed-on: https://chromium-review.googlesource.com/c/1256768Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56433}