Inspected context is created in V8InspectorImpl::contextCreated method and destroyed in V8InspectorImpl::contextDestroyed.
Both methods takes valid v8::Local<v8::Context> handle to the same context, it means that context is created before InspectedContext constructor and is always destroyed after InspectedContext destructor therefore context weak callback in inspected context should be never called.
It's possible only if inspector client doesn't call contextDestroyed which is considered an error.

Therefore CHECK(false) is added into context weak callback to be sure that v8::Context always survives inspected context.

BUG=chromium:652548
R=dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2413583002
Cr-Commit-Position: refs/heads/master@{#40290}