Make ChromeShell activity use AppCompat theme to match official Chrome.

chrome/android/shell/res/values-v17/styles.xml

Index: chrome/android/shell/res/values-v17/styles.xml
diff --git a/chrome/android/shell/res/values-v17/styles.xml b/chrome/android/shell/res/values-v17/styles.xml
new file mode 100644
index 0000000000000000000000000000000000000000..d4b71c20e7e3394d14957a16a02caeb89ab720f0
--- /dev/null
+++ b/chrome/android/shell/res/values-v17/styles.xml
@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<!-- Copyright 2014 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.
+-->
+
+<resources>
+
+    <style name="MainTheme" parent="Theme.AppCompat.Light.NoActionBar">
+        <item name="android:windowContentOverlay">@null</item>
+        <item name="android:windowActionBar">false</item>
+        <item name="android:windowNoTitle">true</item>
+
+        <!-- colorPrimary is used for the default action bar background -->
+        <item name="colorPrimary">@color/material_deep_teal_200</item>
+
+        <!-- colorPrimaryDark is used for the status bar -->
+        <item name="colorPrimaryDark">@color/material_deep_teal_500</item>
+    </style>
+
+</resources>