Allow script timeouts to be optional and associate timeouts with session state
These changes let WebDriver script timeouts be optional and associated all timeout state with the session. Because the durations are currently associated with the handler which is never reset, they bleed across to any subsequent WebDriver sessions.
See each individual commit for more information.
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- [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).
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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because _Servo needs more work before it can pass the WPT WebDriver tests_
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