Support taking screenshots when a webdriver server is active.
Improve the experience of using webdriver to take screenshots of Servo. When a screenshot can't be obtained immediately, increase the delay before trying again from 2ms to 1s.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#13826
- [X] These changes do not require tests because we can't test webdriver yet.
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