[DevTools] Prepare to move Popover to shadow DOM.

third_party/WebKit/Source/devtools/front_end/source_frame/messagesPopover.css

Index: third_party/WebKit/Source/devtools/front_end/source_frame/messagesPopover.css
diff --git a/third_party/WebKit/Source/devtools/front_end/source_frame/messagesPopover.css b/third_party/WebKit/Source/devtools/front_end/source_frame/messagesPopover.css
new file mode 100644
index 0000000000000000000000000000000000000000..f083c465aeeef3eac85736c68739885f436ba9a9
--- /dev/null
+++ b/third_party/WebKit/Source/devtools/front_end/source_frame/messagesPopover.css
@@ -0,0 +1,26 @@
+/*
+ * 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.
+ */
+
+.text-editor-messages-description-container {
+    display: inline-block;
+}
+
+.text-editor-row-message:first-child {
+    border-top-width: 0;
+}
+
+.text-editor-row-message {
+    line-height: 1.2;
+    white-space: nowrap;
+    display: flex;
+    align-items: center;
+    justify-content: flex-start;
+    margin-top: 2px;
+}
+
+.text-editor-row-message-repeat-count {
+    margin-right: 0.5em;
+}