V8PerContextDataHolder should know the world associated with the V8PerContextDataHolder

This is one of incremental steps to clean up code about worlds and contexts.

- Currently V8PerContextDataHolder understands its associated world only if the world is an isolated world. This CL makes V8PerContextDataHolder understand its associated world always.

- This CL adds V8PerIsolateData::m_isMainThread and removes V8PerIsolateData::m_workerDOMDataStore.

BUG=339800

Review URL: https://codereview.chromium.org/146693019

git-svn-id: svn://svn.chromium.org/blink/trunk@166809 bbb929c8-8fbe-4397-9dbb-9b2b20218538