WebUI: Add left/right padding on md-select.

ui/webui/resources/html/md_select_css.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/color.html">
 
 <dom-module id="md-select">
   <template>
     <style>
+      .md-select,
+      .md-select-underline {
+        --md-side-padding: 12px;
+      }
+
       .md-select {
         --md-arrow-width: 0.9em;
         -webkit-appearance: none;
         /* Ensure that the text does not overlap with the down arrow. */
-        -webkit-padding-end: calc(var(--md-arrow-width) * 1.8);
+        -webkit-padding-end: calc(var(--md-side-padding) +
+            var(--md-arrow-width) * 1.8);
+        -webkit-padding-start: var(--md-side-padding);
         background: url(
-          chrome://resources/images/arrow_down.svg) 97% center no-repeat;
+            chrome://resources/images/arrow_down.svg)
+            calc(97% - var(--md-side-padding)) center no-repeat;
         background-size: var(--md-arrow-width);
-        border-bottom: 1px solid var(--paper-grey-300);
-        border-left: none;
-        /* Override Mac's default border-radius */
-        border-radius: 0;
-        border-right: none;
-        border-top: none;
+        border: none;
         color: var(--primary-text-color);
         cursor: pointer;
         font-family: inherit;
         font-size: inherit;
         outline: none;
-        padding-bottom: 3px;
+        padding-bottom: 4px;
         padding-top: 3px;
         width: var(--md-select-width, 200px);
       }
 
       /* Mirroring paper-dropdown-menu disabled style. */
       .md-select[disabled] {
         border-bottom: 1px dashed var(--secondary-text-color);
         color: var(--secondary-text-color);
         opacity: 0.65;
         pointer-events: none;
       }
 
       :host-context([dir=rtl]) .md-select {
-        background-position-x: 3%;
+        background-position-x: calc(var(--md-side-padding) + 3%);
       }
 
+      /* Persistent underline */
       .md-select-underline {
+        border-top: 1px solid var(--paper-grey-300);
+        display: block;
+        margin: 0 var(--md-side-padding);
+      }
+
+      /* Focus underline */
+      .md-select-underline::after {
         border-top: 2px solid var(--google-blue-500);
+        content: '';
         display: block;
+        transform: scale3d(0, 1, 1);
+        transition: transform 200ms ease-in;
+      }
+
+      /* Force the thicker "focus" underline to be properly overlapping with the
+       * "persistent" underline, and also make it visible by 1px while the
+       * <select> is expanded. */
+      .md-select-underline,
+      .md-select-underline::after {
         position: relative;
         top: -1px;
-        transform: scale3d(0, 1, 1);
-        transition: transform 200ms ease-in;
-        width: 100%;
       }
 
-      .md-select:focus + .md-select-underline {
+      .md-select:focus + .md-select-underline::after {
         transform: scale3d(1, 1, 1);
         transition: transform 200ms ease-out;
       }
 
       .md-select-wrapper {
         display: inline-block;
         max-width: 100%;
       }
     </style>
   </template>
 </dom-module>