Make IDBTransaction ctor take ScriptState instead of ExecutionContext

This looks like a no-op, but forthcoming changes to
IDBPendingTransactionMonitor require this and it makes those patches
much easier to understand.

BUG=390704,411532

Review URL: https://codereview.chromium.org/555633002

git-svn-id: svn://svn.chromium.org/blink/trunk@181782 bbb929c8-8fbe-4397-9dbb-9b2b20218538