Add total_length() and NumSlices() methods to QuicMemSliceSpan so that length of HTTP/3 data frame can be computed in QuicSpdyStream::WriteBodySlices().

no behavior change.

Merge internal change: 222285071

Change-Id: Ie3e2a120fc072a9d8329e33fe5cef778aa454c39
Reviewed-on: https://chromium-review.googlesource.com/c/1351584
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Renjie Tang <renjietang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612432}
3 files changed