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This patch adds a new constructor for RefCountedBytes that initializes
it with a given length of zeros and non-const versions of the front()
and front_as<>() methods to accompany the non-const data() method.

This is split out from https://crrev.com/c/786680 which uses this class
as a replacement for net::IOBufferWithSize (but with an unsigned
buffer).

Change-Id: I36c16d02e0a524da3da8679b883321388febaf82
Reviewed-on: https://chromium-review.googlesource.com/822165Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524160}