inspector: turn platform tasks that outlive Agent into no-ops

Turn tasks scheduled on the v8::Isolate or on the given platform into no-ops if the underlying MainThreadInterface has gone away before the task could be run (which would happen when the Environment instance and with it the inspector::Agent instance are destroyed).

This addresses an issue that Electron has been having with inspector support, and generally just seems like the right thing to do, as we may not fully be in control of the relative timing of Environment teardown, platform tasks execution, and the execution of RequestInterrupt() callbacks (although the former two always happen in the same order in our own code).

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines