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

This reverts commit 31f68667.

Reason for revert: Due to fast/forms/text/input-appearance-readonly.html failures on Mac10.13 Tests (dbg).
https://findit-for-me.appspot.com/waterfall/failure?url=https://build.chromium.org/p/chromium.mac/builders/Mac10.13%20Tests%20%28dbg%29/builds/5205

Original change's description:
> 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}

TBR=dmazzoni@chromium.org,alancutter@chromium.org

Change-Id: I4e4270d3d62350bf037b1103ebffd578e819474e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 870062
Reviewed-on: https://chromium-review.googlesource.com/1217645Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590149}

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

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