Khan / perseus / 6149003698

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check for ResizeObserver before using it (#709)

## Summary:
This is basically the same thing they're doing in Webapp, I just didn't notice it before.

Issue: LC-1213

## Test plan:
Not really sure how to write tests for this...maybe I could mock the whole `window` object and just not include `ResizeObserver`? Originally I was going to do something like `window.ResizeObserver = undefined`, but interestingly:

``` JS
window.ResizeObserver = undefined
console.log("ResizeObserver" in window) // true
```

I was able to manually test with `delete`

``` JS
delete window.ResizeObserver
console.log("ResizeObserver" in window) // false
```

Just wasn't sure how to translate that to tests.

Manually testing the regression though:
- Use a mobile web browser or emulate mobile
- Use a browser that doesn't support ResizeObserver (Safari <13.1)
- Go to a page with an [Expression widget](https://www.khanacademy.org/internal-courses/test-everything/test-everything-1/expression/e/expression-exercise)
- Note that rendering the exercise doesn't result in an error

Author: handeyeco

Reviewers: jeremywiebe

Required Reviewers:

Approved By: jeremywiebe

Checks: ✅ finish_coverage, ✅ Publish npm snapshot (ubuntu-latest, 16.x), ✅ Extract i18n strings (ubuntu-latest, 16.x), ✅ Check builds for changes in size (ubuntu-latest, 16.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 16.x), ✅ Check for .changeset file (ubuntu-latest, 16.x), ✅ gerald, ✅ Jest Coverage (ubuntu-latest, 16.x), ✅ Cypress Coverage (ubuntu-latest, 16.x)

Pull Request URL: https://github.com/Khan/perseus/pull/709

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