[ServiceWorker] Add NULL check of ExecutionContext in FetchBodyStream::readAsync

scriptState->executionContext() returns NULL in the following situation.

[Main thread]
- WorkerThread::stop()
 - m_workerGlobalScope->script()->scheduleExecutionTermination();
  - v8::V8::TerminateExecution(m_isolate);

[Worker thread]
- FetchBodyStream::readAsync()
 - ScriptPromiseResolver::create()
  ---> v8::Promise::Resolver::New()
     - v8::internal::Execution::Call
      - v8::internal::Invoke
       - JS
        - v8::internal::__RT_impl_Runtime_StackGuard
         - v8::internal::Isolate::TerminateExecution()
            Sets TerminationException.
 - scriptState->executionContext()
  - toExecutionContext()
   - V8WorkerGlobalScope::findInstanceInPrototypeChain()
    - V8PerIsolateData::findInstanceInPrototypeChain()
     - V8PerIsolateData::findInstanceInPrototypeChain()
      - v8::Object::FindInstanceInPrototypeChain()
       - ON_BAILOUT(isolate,
                    "v8::Object::FindInstanceInPrototypeChain()",
                    return Local<v8::Object>());
        - IsExecutionTerminatingCheck()
          Returns true.

BUG=409755
TEST=run_webkit_tests http/tests/serviceworker/request.html

Review URL: https://codereview.chromium.org/535193002

git-svn-id: svn://svn.chromium.org/blink/trunk@181872 bbb929c8-8fbe-4397-9dbb-9b2b20218538