layout: Use the margin box for vertical positioning of inline-block

fragments if `overflow` is not `visible` per CSS 2.1 § 10.8.1.

Additionally, this patch reverts the change introduced in #12642 in
favor of the spec-compliant behavior described above. This patch also
removes the `inline_block_overflow.html` reftest introduced in #3725, as
the behavior it expected contradicted CSS 2.1 (and in fact the test
fails in Gecko).

The changes that this patch makes to `input_selection_a.html` and
`input_selection_incremental_a.html` are necessary workarounds to make
the tests pass in light of the fact that Servo's UA stylesheet applies
`overflow: hidden` to `<input>` elements. I believe that the changes are
not necessary in other rendering engines because they hard-code
`overflow: hidden`-like behavior for `<input>` elements, while Servo
uses the actual CSS `overflow: hidden` behavior. As far as I can tell,
Servo's behavior is arguably more spec-compliant, but it remains to be
seen how Web compatible it is.

Improves the Google results pages.

Closes #13707.

tests/wpt/metadata-css/css-flexbox-1_dev/html/flexbox_box-clear.htm.ini

@@ -1,3 +0,0 @@
-[flexbox_box-clear.htm]
-  type: reftest
-  expected: FAIL