[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
Don't clip invalidation of composited scrolling contents

The compositor expects that the whole composited scrolling layer to be
invalidated in time and won't update the tiles that are newly visible
when scrolled if the tiles are up-to-date from the compositor side.

If we clip invalidations, the compositor will think the layer areas
outside of the clip are all valid and won't repaint them.

BUG=412516
TEST=compositing/repaint/should-not-clip-composited-viewport-scrolling-layer.html
TEST=compositing/repaint/should-not-clip-composited-overflow-scrolling-layer.html

Review URL: https://codereview.chromium.org/566433002

git-svn-id: svn://svn.chromium.org/blink/trunk@181873 bbb929c8-8fbe-4397-9dbb-9b2b20218538