Don't create debug context if debug listener is not set

Don't create debug context if debug listener is not set

If there had been no debug listener v8::Debug::GetDebugContext would have created new context and wouln't have kept reference to it. This way we may well end up with several debug contexts and disabled debugger.

As a side effect this change allows to efficiently distinguish debug context from blink contexts by simply comparing handles.

BUG=chromium:482290
LOG=Y

Review URL: https://codereview.chromium.org/1136733002

Cr-Commit-Position: refs/heads/master@{#28356}