[Signin] Disable sign in button drop shadow

chrome/android/java/res/layout/account_signin_view.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2015 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.
 -->
 <org.chromium.chrome.browser.signin.AccountSigninView
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:chrome="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
@@ skipping 227 matching lines @@
 
         <!--suppress ButtonStyle -->
         <org.chromium.ui.widget.ButtonCompat
             android:id="@+id/positive_button"
             android:layout_width="wrap_content"
             android:layout_height="36dp"
             android:text="@string/choose_account_sign_in"
             android:textAllCaps="true"
             android:textColor="@android:color/white"
             android:textSize="14sp"
-            chrome:buttonColor="@color/light_active_color"/>
+            chrome:buttonColor="@color/light_active_color"
+            chrome:buttonRaised="false"/>
 
         <View
             android:id="@+id/positive_button_end_padding"
             android:layout_width="0dp"
             android:layout_height="0dp"
             android:layout_weight="1"
             android:visibility="gone"/>
     </LinearLayout>
 
 </org.chromium.chrome.browser.signin.AccountSigninView>