[DevTools] Flip chrome_devtools_session.cc to binary, for good.

chrome_devtools_session has a pointer to the client, which may
request that responses and notifications are provided as JSON.
I'd like to switch this to serialization via CBOR, much like
content/browser/devtools/devtools_session.cc already does it.
Unfortunately this means that I need to transcode here.

However, this should pave the path for removing the json support
from Serializable, as it looks like this is the last remaining
holdout.

Change-Id: I8e4ef2d396973fa30a3bc64a4baa98d0db38a5ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666464
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670688}