<input type=time> with datalist triggers !IsValid() DCHECK

1) Only perform validity checks on truly plain text fields.
   We considered removing the check, but it still has value in terms
   of catching situations where the text field structure changes,
   which have occurred in the past and caused regressions.
2) Remove BrowserAccessibility::IsValid() from non-DCHECK builds.

Bug: 1221781
Cq-Include-Trybots: luci.chromium.try:linux-blink-web-tests-force-accessibility-rel
Change-Id: I5719fa84950f038f19ebe9c616db5946c52e0242

accessibility/crashtests/input-time-datalist.html

@@ -0,0 +1,2 @@
+<input type=time list="suggestions">
+<datalist id=suggestions>