add option for original domain

proto/dom_distiller.proto

Index: proto/dom_distiller.proto
diff --git a/proto/dom_distiller.proto b/proto/dom_distiller.proto
index 54a868d20bea5f7a8ca2a1720b06d4c6c97b3dda..62673955c82510b72574305d714cb90b6b0a82ec 100644
--- a/proto/dom_distiller.proto
+++ b/proto/dom_distiller.proto
@@ -85,4 +85,8 @@ message DomDistillerOptions {
   // (2): (1) and some node visibility information
   // (3): (2) and extracted paging information
   optional int32 debug_level = 2;
+
+  // The original domain of the page if it's a file://, used for detecting next/prev page links
+  // which expects the same domain for both current page and paging links.
+  optional string original_domain = 3;
 }