MD WebUI: Extract settings-dialog into shared cr-dialog element

ui/webui/resources/cr_elements/shared_style_css.html

+<link rel="import" href="chrome://resources/html/polymer.html">
+
+<!-- Common styles for Material Design WebUI. Included directly in
+  settings_shared_css.html. -->
+<dom-module id="cr-shared-style">
+  <template>
+    <style>
+      .action-button {
+        background: var(--google-blue-500);
+        color: white;
+        --paper-button-flat-keyboard-focus: {
+          background-color: rgb(58, 117, 215);  /* 88% of --google-blue-500 */
+        };
+      }
+
+      .action-button[disabled] {
+        opacity: .25;  /* TODO(dbeam): check this value with bettes. */
+      }
+
+      .cancel-button {
+        --paper-button-flat-keyboard-focus: {
+          background-color: rgba(0, 0, 0, .12);
+        };
+      }
+
+      .action-button,
+      .cancel-button {
+        font-weight: 500;
+      }
+    </style>
+  </template>
+</dom-module>