s/IframeLoadEventSteps/IFrameLoadEventSteps/g
All the other IFrame* names have the capital "I" and capital "F", so I'm changing this to keep uniformity.
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
- [ ] These changes do not require tests because it's just a name change and the code is already tested.
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