Add constructors for IOBuffer that take the buffer length as a size_t.

Add constructors for IOBuffer that take the buffer length as a size_t.

Once the consumers have all been updated to work with unsigned buffer lengths, I will remove the signed int versions. For now this change adds some runtime safety checks, and facilitates the ongoing conversion to size_t.

A minority of consumers were calling IOBuffer with something other than "int" or "size_t", and those were updated by this change.

BUG=488553,491315
TBR=michaeln@chromium.org,pfeldman@chromium.org,bradnelson@chromium.org, rockot@chromium.org, dimich@chromium.org

Review URL: https://codereview.chromium.org/1147333003

Cr-Commit-Position: refs/heads/master@{#331485}