test_set_error in test_stylesheet.py is flaky

[The-Compiler]

Something around 460bd86 broke this, see e.g. https://travis-ci.org/qutebrowser/qutebrowser/jobs/358119004:

[...]
----------------------------- Captured stderr call -----------------------------
Qt exceptions in virtual methods:
________________________________________________________________________________
Traceback (most recent call last):
  File "/home/user/qutebrowser.git/qutebrowser/browser/webengine/webview.py", line 292, in javaScriptConsoleMessage
    shared.javascript_log_message(level_map[level], source, line, msg)
  File "/home/user/qutebrowser.git/qutebrowser/browser/shared.py", line 146, in javascript_log_message
    logger(logstring)
  File "/usr/lib64/python3.6/logging/__init__.py", line 1335, in error
    self._log(ERROR, msg, args, **kwargs)
  File "/usr/lib64/python3.6/logging/__init__.py", line 1442, in _log
    self.handle(record)
  File "/usr/lib64/python3.6/logging/__init__.py", line 1452, in handle
    self.callHandlers(record)
  File "/usr/lib64/python3.6/logging/__init__.py", line 1514, in callHandlers
    hdlr.handle(record)
  File "/usr/lib64/python3.6/logging/__init__.py", line 863, in handle
    self.emit(record)
  File "/home/user/qutebrowser.git/tests/helpers/logfail.py", line 51, in emit
    record.getMessage()))
  File "/home/user/qutebrowser.git/.tox/py36/lib/python3.6/site-packages/_pytest/outcomes.py", line 92, in fail
    raise Failed(msg=msg, pytrace=pytrace)
Failed: Got logging message on logger js with level ERROR: [:1] Uncaught TypeError: Failed to execute 'getComputedStyle' on 'Window': parameter 1 is not of type 'Element'.!
________________________________________________________________________________
------------------------------ Captured log call -------------------------------
config.py                  287 DEBUG    Config option changed: content.user_stylesheets = /home/user/qutebrowser.git/tests/unit/javascript/stylesheet/green.css
browsertab.py              732 DEBUG    Predicted navigation: file:///home/user/qutebrowser.git/tests/unit/javascript/non-existent.html
browsertab.py              732 DEBUG    Predicted navigation: file:///home/user/qutebrowser.git/tests/unit/javascript/non-existent.html
objreg.py                  121 DEBUG    schedule removal: 21
objreg.py                  121 DEBUG    schedule removal: tab
objreg.py                  121 DEBUG    schedule removal: hintmanager
objreg.py                  126 DEBUG    removed: 21
objreg.py                  126 DEBUG    removed: tab
objreg.py                  126 DEBUG    removed: hintmanager
browsertab.py              704 DEBUG    load status for <qutebrowser.browser.webengine.webenginetab.WebEngineTab tab_id=22 url='file:///home/user/qutebrowser.git/tests/unit/javascript/non-existent.html'>: LoadStatus.loading
browsertab.py              756 DEBUG    navigation request: url file:///home/user/qutebrowser.git/tests/unit/javascript/non-existent.html, type Type.typed, is_main_frame True
browsertab.py              704 DEBUG    load status for <qutebrowser.browser.webengine.webenginetab.WebEngineTab tab_id=22 url='file:///home/user/qutebrowser.git/tests/unit/javascript/non-existent.html'>: LoadStatus.success
shared.py                  146 INFO     [:1] null

It looks like document.documentElement is null at the point the JS is getting run. I've tried some ideas to stabilize it, but they didn't help. Probably the JS running needs to be delayed a bit more...

cc @jgkamat, though I broke it, so feel free to ignore it 😉