Allow file:// payload urls on non-Brillo boards.

Non-Brillo targets will initially provide payloads from a local file
plus an offset and size. This patch allows file:// URIs on those
targets.

Bug: 25631949
TEST=`update_engine_client --update --follow --payload=file:///some/file`

Change-Id: I90f807b385b9c39ede80faa335e61388a06d58ea
diff --git a/common/libcurl_http_fetcher.h b/common/libcurl_http_fetcher.h
index 900c973..66dbb18 100644
--- a/common/libcurl_http_fetcher.h
+++ b/common/libcurl_http_fetcher.h
@@ -165,6 +165,9 @@
   // Sets the curl options for HTTPS URL.
   void SetCurlOptionsForHttps();
 
+  // Sets the curl options for file URI.
+  void SetCurlOptionsForFile();
+
   // Convert a proxy URL into a curl proxy type, if applicable. Returns true iff
   // conversion was successful, false otherwise (in which case nothing is
   // written to |out_type|).