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A common task when parsing encoded structs from a network is to
read a serialization of an object with (possibly nested) length-prefixed
members.

//third_party/boringssl's Crypto ByteString bakes
support for this in by having a "length-prefixed read" API that
initially reads a big-endian length L from the buffer, then constructs
and returns a new reader on the L many bytes immediately following
the length, skipping the parent reader over this substring. However,
this API is C-style and, as a consequence, perhaps not suitable for
wide use in Chromium code.

This patch adds similar functionality to base::BigEndianReader
by adding ReadU8LengthPrefixed and ReadU16LengthPrefixed methods.

R=mark

Change-Id: Idaf0b04b414a3484099a861ef02501e5f8a83a47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860518
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706074}