Decompose //extensions/browser/BUILD.gn

extensions/browser/api/document_scan/BUILD.gn

+# 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.
+source_set("document_scan") {
+  sources = [
+    "document_scan_api.cc",
+    "document_scan_api.h",
+    "document_scan_interface.cc",
+    "document_scan_interface.h",
+    "document_scan_interface_chromeos.cc",
+  ]
+
+  deps = [
+    "//content/public/browser",
+    "//skia",
+  ]
+
+  if (!is_chromeos) {
+    sources += [ "document_scan_interface_nonchromeos.cc" ]
+  }
+}