DevTools: manage String and ProtocolMessage types independently.

Bug: 929862
Change-Id: I907b342e2503b85fea6e2fbfd3cbbfed1a11db67
Reviewed-on: https://chromium-review.googlesource.com/c/1464426
Reviewed-by: Johannes Henkel <johannes@chromium.org>
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631028}
34 files changed