Incognito context

[aha-oretama]

• forked from https://github.com/GoogleChrome/rendertron/pull/596
• The above PR has two issues, then I fixed them as a new pull request.
  • add ctx.close(), otherwise the memory usage will not be released 💦
  • add same logic into screenshot method

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[aha-oretama]

@googlebot I consent.

[google-cla[bot]]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

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[aha-oretama]

The CI issue is fixed in this PR. The error itself is not related to this PR 😅