[Signin Error Dialog] (1/3) Adds necessary web components

chrome/browser/resources/signin/signin_shared_css.html

+<!-- Common styles for signin-related tab modal dialogs. -->
+<dom-module id="signin-dialog-shared">
+  <template>
+    <style>
+      a {
+        color: var(--google-blue-700);
+        text-decoration: none;
+      }
+
+      body {
+        margin: 0;
+        padding: 0;
+      }
+
+      paper-button {
+        font-weight: 500;
+        line-height: 16px;
+        margin: 0;
+        padding: 8px 16px;
+      }
+
+      paper-button.primary-action {
+        background: var(--google-blue-500);
+        color: white;
+        --paper-button-flat-keyboard-focus: {

[Dan Beam, 2016/09/02 23:21:55]

nit: move to top

[anthonyvd, 2016/09/06 14:36:14]

Done.

+          background: rgb(58, 117, 215);
+        };
+      }
+
+      paper-button.secondary-action {
+        color: var(--paper-grey-600);
+        --paper-button-flat-keyboard-focus: {
+          background: rgba(0, 0, 0, .12);
+        };
+      }
+
+      .container {
+        background-color: white;
+        color: #333;
+        overflow: hidden;
+        width: 448px;
+      }
+
+      .top-title-bar {
+        align-items: center;
+        border-bottom: 1px solid var(--paper-grey-300);
+        display: flex;
+        font-size: 16px;
+        height: 52px;
+        padding: 0 24px;
+      }
+
+      .action-container {
+        display: flex;
+        justify-content: flex-end;
+        padding: 16px;
+      }
+
+<if expr="is_macosx or is_linux">
+      .action-container {
+        flex-flow: row-reverse;
+        justify-content: flex-start;
+      }
+</if>
+
+    </style>
+  </template>
+</dom-module>