Make ChromeShell activity use AppCompat theme to match official Chrome.

chrome/android/shell/java/AndroidManifest.xml

 <?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.
  -->
 
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="org.chromium.chrome.shell">
@@ skipping 25 matching lines @@
     <permission android:name="org.chromium.chrome.shell.permission.DEBUG"
                 android:label="Debug web pages in Chrome Shell"
                 android:permissionGroup="android.permission-group.DEVELOPMENT_TOOLS"
                 android:protectionLevel="signature" />
 
     <application android:name="org.chromium.chrome.shell.ChromeShellApplication"
             android:icon="@mipmap/app_icon"
             android:label="Chrome Shell">
         <activity android:name="org.chromium.chrome.shell.ChromeShellActivity"
                   android:launchMode="singleTask"
-                  android:theme="@android:style/Theme.Holo.Light.NoActionBar"
+                  android:theme="@style/MainTheme"
                   android:configChanges="orientation|keyboardHidden|keyboard|screenSize"
                   android:hardwareAccelerated="true">
               <intent-filter>
                   <action android:name="android.intent.action.MAIN" />
                   <category android:name="android.intent.category.LAUNCHER" />
               </intent-filter>
          </activity>
         <activity android:name="org.chromium.sync.test.util.MockGrantCredentialsPermissionActivity"
                   android:exported="true">
             <intent-filter>
@@ skipping 96 matching lines @@
         <receiver android:name="org.chromium.chrome.shell.signin.AccountsChangedReceiver">
             <intent-filter>
                 <action android:name="android.accounts.LOGIN_ACCOUNTS_CHANGED" />
             </intent-filter>
         </receiver>
         <meta-data android:name="org.chromium.content.browser.SMART_CLIP_PROVIDER"
                    android:value="org.chromium.content.browser.SmartClipProvider" />
 
     </application>
 </manifest>