DevTools: implement release note behind an experiment

third_party/WebKit/Source/devtools/front_end/help/releaseNote.css

+/*
+ * Copyright 2017 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+.release-note-header {
+    font-size: 15px;
+}
+
+.release-note-container {
+    margin: 12px;
+    display: flex;
+    flex-direction: row;
+    flex-wrap: wrap-reverse;
+    align-content: flex-start;
+}
+
+.release-note-highlight-container {
+    margin: 12px 0 12px -25px;
+    line-height: 1.5;
+    width: 100%;
+}
+
+.release-note-link, .release-note-featured-link {
+    color: #039be5;
+    cursor: pointer;
+}
+
+.release-note-link:hover,
+.release-note-featured-link:hover,
+.image-hover .release-note-featured-link {
+    text-decoration: underline;
+}
+
+.release-note-image-container {
+    cursor: pointer;
+    padding: 0 0 12px 0;
+}
+
+.release-note-image {
+    width: 200px;
+}
+
+.release-note-text-container {
+    width: 300px;
+}
+
+.close-release-note {
+    margin-left: 4px;
+}
+
+/* Buttons and dropdowns */
+.dth-button {
+    -webkit-appearance: none;
+    box-sizing: border-box;
+    outline: 0;
+    height: 24px;
+    font-family: ".SFNSDisplay-Regular", "Segoe UI", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif;
+    font-size: 12px;
+    line-height: 16px;
+    border: 1px solid;
+    border-radius: 2px;
+    padding: 3px 12px;
+    font-weight: 600;
+    color: #333;
+    background-color: #fff;
+    border-color: rgba(0, 0, 0, 0.2);
+}
+
+.dth-button:hover, .dth-button:focus, .dth-button:active {
+    background-color: #fafafa;
+    box-shadow: 0 1px 2px rgba(0, 0, 0, 0.1);
+}
+
+.dth-button:active {
+    background-color: #f2f2f2;
+}
+
+.dth-button:focus {
+    box-shadow: 0 1px 2px rgba(0, 0, 0, 0.1), 0 0 0 2px rgba(66, 133, 244, 0.4);
+}
+
+.dth-button.is-primary {
+    background-color: #4285F4;
+    border: 0;
+    color: #fff;
+}
+
+.dth-button.is-primary:hover, .dth-button.is-primary:focus {
+    background-color: #3B78E7;
+}