Corrected case of Document#characterSet

Resolves #4251
Use .clone() instead of.as_slice().to_string()
Update content tests for Document#characterSet
Implement the "messageevent" argument to Document#createEvent. #4008
Remove an expected failure from tests/wpt/metadata/dom/events/EventTarget-dispatchEvent.html.ini
auto merge of #4265 : ProgramFOX/servo/issue-4251, r=Ms2ger

Resolves #4251
Use or_init where appropriate.
auto merge of #4263 : Ms2ger/servo/or_init, r=jdm
Fix breakage of `./mach test`

When running commands through Registrar.dispatch, mach does not behave
in the same way it would as if it were running through the command line.
Defaults normally provided through a combination of @CommandArgument and
argparse magic are ignored.

I have some ideas as to how to fix this, but until then, this will allow
`./mach test` to run through test-unit properly.
auto merge of #4269 : mttr/servo/mach_test_fix, r=mbrubeck

When running commands through Registrar.dispatch, mach does not behave
in the same way it would as if it were running through the command line.
Defaults normally provided through a combination of @CommandArgument and
argparse magic are ignored.

I have some ideas as to how to fix this, but until then, this will allow
`./mach test` to run through test-unit properly.
servo.css: Style <button> like <input type=button>
Fix path for CEF build.
Update cargo.lock for CEF and fix left over line from libgreen removal.
auto merge of #4207 : glennw/servo/fix-cef, r=larsbergstrom
auto merge of #4277 : mttr/servo/style_button_element, r=SimonSapin
auto merge of #4257 : johnathan79717/servo/4008, r=jdm

This is a fix for issue #4008.
auto merge of #4266 : Ms2ger/servo/js-document-crash, r=Manishearth

This prevents a crash when the script in the javascript URL accesses the
document attribute.
Simplify the content_changed call in ScriptTask::load.
Cleanup last_loaded_url/last_url handling in ScriptTask::load.
Pass the load data to parse_html directly, rather than in an Option.