Copy a version of download action for ChromeOS

DownloadAction contains many cros specific stuff, such as p2p, system
state. Many of these cros bits are used in a fragile way in aosp. It's
better if we maintain two independent download actions.

Test: treehugger
Bug: 175042161
Change-Id: Iaaed269b3886a068bfcb4bcac12aca99b9cf6970
diff --git a/BUILD.gn b/BUILD.gn
index 1ddae22..e60d33b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -210,7 +210,7 @@
     "cros/requisition_util.cc",
     "cros/shill_proxy.cc",
     "cros/update_attempter.cc",
-    "download_action.cc",
+    "cros/download_action_chromeos.cc",
     "libcurl_http_fetcher.cc",
     "metrics_utils.cc",
     "update_boot_flags_action.cc",
@@ -492,7 +492,7 @@
       "cros/payload_state_unittest.cc",
       "cros/requisition_util_unittest.cc",
       "cros/update_attempter_unittest.cc",
-      "download_action_unittest.cc",
+      "cros/download_action_chromeos_unittest.cc",
       "libcurl_http_fetcher_unittest.cc",
       "metrics_utils_unittest.cc",
       "payload_consumer/bzip_extent_writer_unittest.cc",