Fix crash in ScriptDebugServer::setBreakpoint

The crash was presumably caused by breakpointId handle being null because of exception in debugger. Generally debugger calls shouldn't throw but since some parts of it are implemented in JS there may be a stack overflow which would result in an exception and empty result handle.

BUG=381567

Review URL: https://codereview.chromium.org/341713008

git-svn-id: svn://svn.chromium.org/blink/trunk@176402 bbb929c8-8fbe-4397-9dbb-9b2b20218538