ohos: WebViewControllerImpl: Introduce JsScopeData

Move the WebviewController reference into the established JsScopeData
pattern: a private struct that bundles the JS-thread-affine state behind
a shared_ptr whose deleter releases its contents on the JS thread.

This is a preparatory, behavior-preserving change replacing the lone
m_jsWebViewController member, making room to extend the JS-thread state
in a follow-up change.

Task-number: QTBUG-146838
Pick-to: 6.12
Change-Id: Ic319f9250e3a9f9429615586e1e30afde3f610c3
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Dennis Kim <dennis.kim@qt.io>