DevTools protocol is not supposed to carry structurally invalid utf8 as string payload.

Bug: chromium:929862
Change-Id: I701eeb553e6bf22d887947dcd9f4b29af7a43e2b
Reviewed-on: https://chromium-review.googlesource.com/c/1475665Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59643}