Remove unneeded bottom shadow from bottom sheet

This patch removes an unused bottom shadow from the bottom sheet's
toolbar. Modern design does not require one by default, existing
sheet contents provide their own shadow when needed, and the existing
one caused issues on Android J devices.

Bug: 829454
Change-Id: I4cf9a36e6efbf107499516181f97c57c556632cf
Reviewed-on: https://chromium-review.googlesource.com/1068653Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560838}

chrome/android/java/res/drawable/bottom_toolbar_shadow.xml

-<?xml version="1.0" encoding="utf-8"?>
-<!-- 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. -->
-
-<!-- ToolbarPhone uses this drawable to transition between shadows when
-     the omnibox focus changes. The bottom toolbar should not have a shadow
-     when the omnibox is unfocused so this TransitionDrawable uses an empty
-     shape for the focused drawable item.
-     TODO(twellington): Remove this after UX has a spec'ed out what the
-     bottom toolbar omnibox un/focus animations should look like. -->
-<transition xmlns:android="http://schemas.android.com/apk/res/android">
-    <item>
-        <shape
-            android:shape="rectangle"
-            android:gravity="top|fill_horizontal" >
-        </shape>
-    </item>
-    <item>
-        <bitmap
-            android:src="@drawable/toolbar_shadow_focused"
-            android:gravity="top|fill_horizontal" />
-    </item>
-</transition>

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

@@ -47,14 +47,6 @@
 
     </FrameLayout>
 
-    <ImageView
-        android:id="@+id/bottom_sheet_toolbar_bottom_shadow"
-        android:src="@drawable/bottom_toolbar_shadow"
-        android:layout_width="match_parent"
-        android:layout_height="wrap_content"
-        android:scaleType="fitXY"
-        android:contentDescription="@null" />
-
     <FrameLayout
         android:id="@+id/bottom_sheet_snackbar_container"
         android:layout_width="match_parent"