[DevTools] Add headless::protocol::Binary to protocol_string{.h,cc}.

This is in preparation of referencing this type from the code
generated by third_party/inspector_protocol/code_generator.py /
headless/lib/browser/devtools_api/client_api_generator.py.

My overall WIP change is in here, and happy to explain more:
https://chromium-review.googlesource.com/c/chromium/src/+/1244719

Bug: chromium:891377
Change-Id: I1d053de59f0e6c07cfe42b29a3e09aaab91ac330
Reviewed-on: https://chromium-review.googlesource.com/c/1281166
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600160}
4 files changed