Some of the DevTools' clients need to inspect JS objects without enabling debugger. This CL allows to inspect object's internal properties without enabling debugger and instantiating debug context.

Note that now debug context can be created lazily if v8::Debug::GetDebugContext is called when there is no debug listener. This is fragile and has already resulted in some subtle error. I'm going to fix that in a separate CL.

BUG=chromium:481845
LOG=Y

Review URL: https://codereview.chromium.org/1126103006

Cr-Commit-Position: refs/heads/master@{#28362}