IndexedDB: Change IDBPendingTransactionMonitor's owner from ThreadSpecific to ExecutionContext.

This CL makes ExecutionContext Supplementable, and makes
IDBPendingTransactionMonitor a Supplement<ExecutionContext>.
handles IDBTransaction in its execution context.

ServiceWorkerContainerClient.cpp needs a small change because
Document now has two provideSupplement functions.

This is a preparation to move IDBTransaction to Oilpan heap because
ThreadSpecific and Oilpan don't work well together.

BUG=366528

Review URL: https://codereview.chromium.org/258893009

git-svn-id: svn://svn.chromium.org/blink/trunk@172940 bbb929c8-8fbe-4397-9dbb-9b2b20218538