[DevTools] Allow modifying instrumenting agents while iterating.

In this case we stop iteration early (possibly loosing instrumentation call though).

This can happen if during instrumentation call we paused the execution, entered
nested message loop and detached from there (thus removing agent from the list).

BUG=none

Review-Url: https://codereview.chromium.org/2139443002
Cr-Commit-Position: refs/heads/master@{#404694}
3 files changed