Android: use grit target platform instead of define.

chrome/renderer/resources/plugin_placeholders.css

 /* Copyright (c) 2012 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. */
 
 body {
   font-family: sans-serif;
   margin: 0;
   overflow: hidden;
   text-align: center;
   -webkit-user-select: none;
 }
 
 h1 {
   font-size: 10pt;
   font-weight: normal;
   padding: 0pt 10pt;
-<if expr="not pp_ifdef('android') and not pp_ifdef('chromeos')">
+<if expr="not is_android and not pp_ifdef('chromeos')">
   visibility: hidden;
 </if>
 }
 
 #outer:hover h1, #outer:hover #close {
   visibility: visible;
 }
 
 p {
   font-size: 8pt;
   padding: 0pt 14pt;
 }
 
 #outer {
   border: 1px black solid;
   box-sizing: border-box;
   height: 100%;
   position: absolute;
   width: 100%;
 }
 
 #inner {
   position: relative;
   height: 100%;
   width: 100%;
   top: 50%;
-<if expr="not pp_ifdef('android')">
+<if expr="not is_android">
   margin-top: -70px;
 </if>
-<if expr="pp_ifdef('android')">
+<if expr="is_android">
   margin-top: -14px;
 </if>
 }
 
 #close {
-<if expr="not pp_ifdef('android')">
+<if expr="not is_android">
   visibility: hidden;
 </if>
   cursor: pointer;
   position: absolute;
   right: 3px;
   top: 3px;
   height: 14px;
   width: 14px;
   background-image: -webkit-image-set(
       url('../../../ui/resources/default_100_percent/close_2.png') 1x,
       url('../../../ui/resources/default_200_percent/close_2.png') 2x);
   background-position: right top;
   background-repeat: no-repeat;
 }
 
 #close:hover {
   background-image: -webkit-image-set(
       url('../../../ui/resources/default_100_percent/close_2_hover.png') 1x,
       url('../../../ui/resources/default_200_percent/close_2_hover.png') 2x);
 }
 
 #close:active {
   background-image: -webkit-image-set(
       url('../../../ui/resources/default_100_percent/close_2_pressed.png') 1x,
       url('../../../ui/resources/default_200_percent/close_2_pressed.png') 2x);
 }