Not setting one of them in a SetTimeouts requests should not remove it.
This fixes a panicking `unwrap()`.

When sending a webdriver load URL command soon enough after starting Servo,
that command could time out with a logged warning:

```
constellation: Webdriver load for closed browsing context (0,2).
```

When `closed` in this case really meant not opened yet.

…ding

```
Traceback (most recent call last):
  File "/home/simon/servo1/tests/wpt/web-platform-tests/tools/wptrunner/wptrunner/testrunner.py", line 90, in run
    rv = commands[command](*args)
  File "/home/simon/servo1/tests/wpt/web-platform-tests/tools/wptrunner/wptrunner/testrunner.py", line 110, in wait
    self.executor.wait()
  File "/home/simon/servo1/tests/wpt/web-platform-tests/tools/wptrunner/wptrunner/executors/base.py", line 209, in wait
    self.protocol.base.wait()
AttributeError: 'ServoWebDriverProtocol' object has no attribute 'base'
```

```
 0:27.48 pid:4157 Traceback (most recent call last):
 0:27.48 pid:4157   File "/home/simon/servo1/etc/run_in_headless_android_emulator.py", line 212, in <module>
 0:27.48 pid:4157     sys.exit(main(*sys.argv[1:]))
 0:27.48 pid:4157   File "/home/simon/servo1/etc/run_in_headless_android_emulator.py", line 72, in main
 0:27.48 pid:4157     forward_webdriver(adb, args)
 0:27.48 pid:4157   File "/home/simon/servo1/etc/run_in_headless_android_emulator.py", line 163, in forward_webdriver
 0:27.48 pid:4157     ports = [int(part) for part in split if part]
 0:27.48 pid:4157 ValueError: invalid literal for int() with base 10: 'None'
```

This makes `/_mozilla/mozilla/webgl/context_creation_error.html` pass,
for example.

```
./ mach test-wpt --product servodriver \
  --binary etc/run_in_headless_android_emulator.py \
  --binary-arg servo-x86 \
  --binary-arg target/i686-linux-android/release/servo.apk \
  /_mozilla/mozilla/webgl/context_creation_error.html
```