Simplify InspectorIndexedDBAgent by using NewScriptState::Scope

This CL introduces NewScriptState::forMainWorld() and simplifies InspectorIndexedDBAgent by using NewScriptState::Scope.

BUG=357144

Review URL: https://codereview.chromium.org/238683002

git-svn-id: svn://svn.chromium.org/blink/trunk@171551 bbb929c8-8fbe-4397-9dbb-9b2b20218538