fix:webframe spellcheck client leak for sandbox mode

[nitsakh]

Description of Change

RenderFrameObserver OnDestruct method is only called in non sandbox mode. So spellcheck client was destroyed only in non sandbox mode.
This fixes that and moves the spellcheck client destruction to WillReleaseScriptContext which will be called in both sandbox and non-sandbox mode.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is changed or added
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed webframe spellcheck provider memory leak for sandbox mode

[zcbenz]

I have created #16770 based on the changes here, which should hopefully fix most leak sources.

[nitsakh]

Thanks! I'll close this one then.