android_webview: stop using system ashmem.

third_party/ashmem/BUILD.gn

 # 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.
 
 assert(is_android)
 
-import("//build/config/android/config.gni")
-
-if (is_android_webview_build) {
-  # WebView build uses the Android system version of ashmem to avoid linking
-  # problems.
-  group("ashmem") {
-    assert(false, "Implement system ashmem for webview build")
-  }
-} else {
-  source_set("ashmem") {
-    sources = [
-      "ashmem.h",
-      "ashmem-dev.c",
-    ]
-  }
+source_set("ashmem") {
+  sources = [
+    "ashmem.h",
+    "ashmem-dev.c",
+  ]
 }