Show a focus ring when a readonly text field is focused.

The previous behavior was deliberate but dates back to this 2006
WebKit bug without any clear rationale:
https://bugs.webkit.org/show_bug.cgi?id=8705

In the absence of a strong argument in favor of this behavior,
we should do what's better for users and show the focus ring.

Bug: 870062
Change-Id: I6c1c427d05270a69d4ded74f68704d78b78c3095
Reviewed-on: https://chromium-review.googlesource.com/1213311
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590043}

third_party/WebKit/LayoutTests/fast/forms/text/input-readonly-focus-ring-expected.html

+ <input id="target" readonly style="outline-offset: -2px; outline: -webkit-focus-ring-color auto 5px;">

third_party/WebKit/LayoutTests/fast/forms/text/input-readonly-focus-ring.html

+<input id="target" readonly>
+<script>target.focus()</script>

third_party/blink/renderer/core/css/html.css

@@ -992,8 +992,7 @@ nobr {
     outline: auto 5px -webkit-focus-ring-color
 }
 
-/* Read-only text fields do not show a focus ring but do still receive focus */
-html:focus, body:focus, input[readonly]:focus {
+html:focus, body:focus {
     outline: none
 }