[MD settings] use arrow-forward to switch direction in rtl

chrome/browser/resources/settings/settings_shared_css.html

 <link rel="import" href="chrome://resources/cr_elements/shared_style_css.html">
 <link rel="import" href="/settings_vars_css.html">
 
 <!-- Common styles for Material Design settings. -->
 <dom-module id="settings-shared">
   <template>
     <style include="cr-shared-style">
       button[is='paper-icon-button-light'] {
         --paper-icon-button-light-ripple: {
           /* Center the ripple on the icon button. */
-          height: 36px;
+          height: var(--settings-icon-ripple-size);
           left: -8px;
           top: -8px;

[Dan Beam, 2016/10/03 23:42:06]

why can't we just write the code in a direction-agnostic way?  i.e. margin: 8px?

[dschuyler, 2016/10/04 00:56:27]

Acknowledged.

-          width: 36px;
+          width: var(--settings-icon-ripple-size);
         };
-        -webkit-margin-start: 16px;
+        /* Not using -webkit-margin-start because we mirror the control in rtl.

[Dan Beam, 2016/10/03 23:42:06]

why not?  it'd be one less rule you have to duplicate or override

[dschuyler, 2016/10/04 00:56:27]

Acknowledged.

+         */
+        margin-left: 16px;
         background-size: cover;
         flex-shrink: 0;
-        height: 20px;
-        width: 20px;
+        height: var(--settings-icon-size);
+        width: var(--settings-icon-size);
       }
 
-      [actionable] button[is="paper-icon-button-light"].icon-arrow-right {
+      :host-context([dir=rtl]) button[is='paper-icon-button-light'] {
+        --paper-icon-button-light-ripple: {
+          /* Center the ripple on the icon button. */
+          height: var(--settings-icon-ripple-size);
+          right: -8px;
+          top: -8px;
+          width: var(--settings-icon-ripple-size);
+        };
+        transform: scaleX(-1); /* Flip on the X axis (aka mirror). */
+        /* -webkit-margin-start can be upset by the transform; manually control

[Dan Beam, 2016/10/03 23:42:06]

wait, what?  is this a transform-origin issue?

[dschuyler, 2016/10/04 00:56:27]

Acknowledged.

+         * the margin. */
+        margin-left: 0;
+        margin-right: 16px;
+      }
+
+      [actionable] button[is="paper-icon-button-light"].icon-arrow-forward {
         background-image: url(images/arrow_right.svg);
       }
 
       [actionable] button[is="paper-icon-button-light"].icon-external {
         background-image: url(images/open_in_new.svg);
       }
 
       /* Use <h2> as the "sub-header" mentioned in the UX design docs. */
       h2 {
         align-items: center;
@@ skipping 390 matching lines @@
       /* Turns the arrow direction downwards, when the bubble is placed above
        * the anchor element */
       .search-bubble-innards.above::after {
         -webkit-transform: rotate(-135deg);
         bottom: -5px;
         top: auto;
       }
     </style>
   </template>
 </dom-module>