Drop V8RecursionScope dependency on ExecutionContext

To support using V8RecursionScope in other places, drop the dependency on ExecutionContext in favor of just an v8::Isolate. This requires moving IDBPendingTransactionMonitor from a supplement on the context to per-isolate data.

Review URL: https://codereview.chromium.org/429453010

git-svn-id: svn://svn.chromium.org/blink/trunk@181946 bbb929c8-8fbe-4397-9dbb-9b2b20218538