GitLab

- Adds a custom BigIOBuffer subclass of net::IOBufferWithSize,
  backed by a mojo_base::BigBuffer.
- Uses this buffer to avoid a copy for large code data (> 64KB). In
  order to avoid a copy, the disk entry is read in two parts, response
  time and code.
- Small code data (< 64 KB) uses a single read and copies as before.

Bug: chromium:992991
Change-Id: I520c8983b04c323e8ea9c8d817f94a3a64b4f7c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1769084Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691658}