Fix crash in textinput
Implements HTMLInputElement.defaultValue

...and changes SetValue to update the input text instead of the content
attr.

Also includes a comment summarizing everything I currently know with
respect to an input elements checkedness vs its IDL attributes vs its
content attributes.
Implements basic form resetting

What can this do? Reset `<input type=text>` fields back to their default
value through a call to a form's reset method. That's all for now!

Fixes compile error after rebase
Adds reset method to FormControl
Implements FormControl for HTMLTextAreaElement
Implements reset button
htmltextarea: Fixed some value_changed issues

Also modified tests/html/textarea.html to allow for the testing of the
textarea's dirty value flag.
Implements DefaultChecked and resets of checkboxes
Implements dirty value/checked flags for input

And modifies test-inputs.html to test.

Fixes wpt breaking mistake
Updated WPT to expect newly passing tests
Implements :indeterminate pseudo-class

Addresses reviews
Inset and outset border coloring is incorrect on left side border.

This patch will fix this issue.
Thanks to differences on rendering between CPU X GPU, there is a 1 pixel wide
difference in the position where the top border meets the right border (as
also the right border X bottom border).

The workaround to have a test that will work on both modes is to cover the
affected area with a white div. This patch adds inset test.
Adding tests for outset.
Additional updates for newly passing WPT's
auto merge of #4133 : mttr/servo/form_resetting, r=jdm

We can reset `<input type=text>` fields! I wish I could've done something with checkboxes, but unfortunately, that's it for now.

In addition to that, this PR implements `HTMLInputAttribute.defaultValue`, updates wpt-test to expect passing tests as a result of that implementation, and fixes an index error crash with text inputs.

edit: also includes an html example where one may lazily watch form resets in action: ` tests/html/form_reset_handsfree.html`
handle null for evaluating JS value. #4235
auto merge of #4397 : jdm/servo/issue_4235, r=jdm

Rebased from #4241.
Cropping the ref images to save some bytes (as also adjust test borders
to match results).
auto merge of #4393 : Adenilson/servo/fixInsetOutsetColor01, r=pcwalton