Refactor html form dataset collection

[anthgur]

Factor out FormDatum collection for <input>
Improve early return logic for getting the FormDatum from an <input>
Condense element type patterns

Proposed to close #7851

[jdm]

Great work! Thanks for fixing the TODO about button submitters, too :) Just a few changes required to merge this now.
-S-awaiting-review +S-needs-code-changes

---

Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, 5 unresolved discussions, all commit checks successful.

---

^{components/script/dom/htmlformelement.rs, line 234 [r1] (raw file):}
This comment belongs above the traverse_preorder call below, like in the original code.

---

^{components/script/dom/htmlformelement.rs, line 238 [r1] (raw file):}
Let's move this comment to the line following the .collect() like in the original code.

---

^{components/script/dom/htmlformelement.rs, line 244 [r1] (raw file):}
Please indent this like the original code.

---

^{components/script/dom/htmlformelement.rs, line 246 [r1] (raw file):}
nit: this should be at the same indentation level as the if

---

^{components/script/dom/htmlinputelement.rs, line 452 [r1] (raw file):}
I think this code could be written more clearly as:

if ty == "radio" || ty == "checkbox" {
    if value.is_empty() {
        value = "on".to_owned();
    }
}
Some(FormDatum {
    ty: ty,
    name: name,
    value: value,
})

---

Comments from the review on Reviewable.io

[anthgur]

@jdm thanks, fixed! Had to enclose the chained conditionals in parens for the compiler.

[jdm]

-S-awaiting-review +S-needs-code-changes

---

Reviewed 2 of 2 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

---

^{components/script/dom/htmlinputelement.rs, line 437 [r2] (raw file):}
This should be ||, and that will allow us to remove the ()s.

---

Comments from the review on Reviewable.io

[anthgur]

@jdm ah, is that a bitwise or when it's not in a match block?

[jdm]

Correct.

[jdm]

Looks good! Go ahead and squash the commits into one!
-S-awaiting-review +S-needs-squash

---

Reviewed 1 of 1 files at r3.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

---

Comments from the review on Reviewable.io

[jdm]

@bors-servo: r+

[bors-servo]

📌 Commit 31980a7 has been approved by jdm

[bors-servo]

⌛ Testing commit 31980a7 with merge 94dc987...

[bors-servo]

💔 Test failed - mac-rel-css

[jdm]

@bors-servo: retry

• #7910

[bors-servo]

⚡ Previous build results for android, gonk, linux-dev, linux-rel, mac-dev-ref-unit, mac-rel-wpt are reusable. Rebuilding only mac-rel-css...

[bors-servo]

☀️ Test successful - android, gonk, linux-dev, linux-rel, mac-dev-ref-unit, mac-rel-css, mac-rel-wpt