WebUI: Improve calculation of md-select text and arrow spacing.

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;
+
+        /* The offset of the arrow from the end of the underline. */
+        --md-arrow-offset: 3%;
+
         -webkit-appearance: none;
         -webkit-margin-end: calc(-1 * var(--md-side-padding));
-        /* Ensure that the text does not overlap with the down arrow. */
+        /* Ensure that there is a 3px space between the text and the arrow. */
         -webkit-padding-end: calc(var(--md-side-padding) +
-            var(--md-arrow-width) * 1.8);
+            var(--md-arrow-offset) + var(--md-arrow-width) + 3px);
         -webkit-padding-start: var(--md-side-padding);
         background: url(
             chrome://resources/images/arrow_down.svg)
-            calc(97% - var(--md-side-padding)) center no-repeat;
+            calc(100% - var(--md-arrow-offset) - var(--md-side-padding))
+            center no-repeat;
         background-size: var(--md-arrow-width);
         border: none;
         color: var(--primary-text-color);
         cursor: pointer;
         font-family: inherit;
         font-size: inherit;
         outline: none;
         padding-bottom: 4px;
         padding-top: 3px;
         width: calc(var(--md-select-width, 200px) + 2 * var(--md-side-padding));
       }
 
       /* Mirroring paper-dropdown-menu disabled style. */
       .md-select[disabled] {
         pointer-events: none;
       }
 
       .md-select[disabled],
       .md-select[disabled] + .md-select-underline {
         color: var(--secondary-text-color);
         opacity: 0.65;
       }
 
       :host-context([dir=rtl]) .md-select {
-        background-position-x: calc(var(--md-side-padding) + 3%);
+        background-position-x: calc(var(--md-side-padding) +
+            var(--md-arrow-offset));
       }
 
       /* Persistent underline */
       .md-select-underline {
         -webkit-margin-end: 0;
         -webkit-margin-start: var(--md-side-padding);
         border-top: 1px solid var(--paper-grey-300);
         display: block;
         margin-bottom: 0;
         margin-top: 0;
@@ skipping 26 matching lines @@
         border-top: 1px dashed var(--secondary-text-color);
       }
 
       .md-select-wrapper {
         display: inline-block;
         max-width: 100%;
       }
     </style>
   </template>
 </dom-module>