Fix memory leak when using webFrame and spell checker

[zcbenz]

Description of Change

Close #15459.

When using spell checker in pages with sandbox, there are two sources of memory leak:

1. Every time when page is reloaded, the code would create a new api::WebFrame, and the old one would be leaked since the renderer process is not restarted.
2. The spell check client is only destroyed when web frame is destroyed, but reloading a frame does not destroy the corresponding blink::WebFrame.

This PR fixes the leak with 2 methods:

1. Refactor the webFrame module so it does not create api::WebFrame instances, the native methods become simple getters without a global state. So no memory will be leaked on reload, and the code now is cleaner and uses less memory.
2. The spell check client is now destroyed when the page context is destroyed, so it won't hold a reference to code in the old context, which prevents garbage collection of V8.

Note that this PR should be rebased instead of squashed as the commits have clear responsibilities.

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: Fix memory leak when using webFrame and spell checker.

[nitsakh]

LGTM! 👍 Thanks!

[release-clerk]

Release Notes Persisted

Fix memory leak when using webFrame and spell checker.

[trop]

I have automatically backported this PR to "5-0-x", please check out #16851