Simplify view-source line-wrapping implementation

Previously, we created a wrapper <div>, added an `id` attribute to
the <input type=checkbox>, and added the corresponding `for`
attribute to the <label>. This patch simplifies the implementation
as follows:

- We get rid of both the `id` and `for` attributes by wrapping the
  checkbox and the text label inside of the <label>.
- We then also get rid of the wrapper <div>.

This simplifies the code and reduces the resulting DOM size.

This patch also improves the UI: now, the entire horizontal bar can
be clicked to toggle the checkbox, as opposed to just the checkbox
itself + the text label (whose length is variable, based on the
user’s locale + font sizing/zoom preferences).

Tests:

    autoninja -C out/Release blink_unittests
    out/Release/blink_unittests --gtest_filter=HTMLViewSourceDocumentTest.*

Bug: chromium:920571
Change-Id: I42873f62d94b264a1b7edf3567e539301be07ed1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2598907
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840782}

third_party/blink/renderer/core/html/html_view_source_document.cc

@@ -108,25 +108,21 @@ void HTMLViewSourceDocument::CreateContainingTable() {
   line_number_ = 0;
 
   // Create a checkbox to control line wrapping.
-  auto* line_wrap_div = MakeGarbageCollected<HTMLDivElement>(*this);
-  line_wrap_div->setAttribute(html_names::kClassAttr, "line-wrap-control");
+  auto* label = MakeGarbageCollected<HTMLLabelElement>(*this);
+  label->setAttribute(html_names::kClassAttr, "line-wrap-control");
   auto* checkbox =
       MakeGarbageCollected<HTMLInputElement>(*this, CreateElementFlags());
   checkbox->setAttribute(html_names::kTypeAttr, "checkbox");
-  checkbox->setAttribute(html_names::kIdAttr, "line-wrap-checkbox");
-  line_wrap_div->ParserAppendChild(checkbox);
-  auto* label = MakeGarbageCollected<HTMLLabelElement>(*this);
-  label->setAttribute(html_names::kForAttr, "line-wrap-checkbox");
+  label->ParserAppendChild(checkbox);
   label->ParserAppendChild(
       Text::Create(*this, WTF::AtomicString(Locale::DefaultLocale().QueryString(
                               IDS_VIEW_SOURCE_LINE_WRAP))));
-  line_wrap_div->ParserAppendChild(label);
   auto* tr = MakeGarbageCollected<HTMLTableRowElement>(*this);
   auto* td =
       MakeGarbageCollected<HTMLTableCellElement>(html_names::kTdTag, *this);
   td->setAttribute(html_names::kColspanAttr, "2");
   td->setAttribute(html_names::kClassAttr, "line-wrap-cell");
-  td->ParserAppendChild(line_wrap_div);
+  td->ParserAppendChild(label);
   tr->ParserAppendChild(td);
   tbody_->ParserAppendChild(tr);