Don't silently eat Inspector.TargetCrashed

For the browser test and headless_shell the devtools event is used
to detect renderer crashes.  We retain the C++ observer since it
returns the termination reason (not avaliable over devtools)

BUG=546953

Review-Url: https://codereview.chromium.org/2514343002
Cr-Commit-Position: refs/heads/master@{#433903}
3 files changed