Re-land: Show a focus ring when a readonly text field is focused.

Original: crrev.com/c/1213311
Revert: crrev.com/c/1217645

(Issue was just with a mac10.13 expectation)

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.

TBR=alancutter@chromium.org

Bug: 870062
Change-Id: I2d413cec6cd94528f5f99852896a99ea68b4bdb7
Reviewed-on: https://chromium-review.googlesource.com/1220169Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590558}

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
 }