Replace source pointer in cc::CopyOutputRequest with a base::UnguessableToken

cc/output/copy_output_request.cc

Index: cc/output/copy_output_request.cc
diff --git a/cc/output/copy_output_request.cc b/cc/output/copy_output_request.cc
index 05f15ae1727c1c51c0a77221a912f4b7cfa54616..64ada7f63c8c30ae699c41e3703c697eb49db77b 100644
--- a/cc/output/copy_output_request.cc
+++ b/cc/output/copy_output_request.cc
@@ -28,16 +28,14 @@ std::unique_ptr<CopyOutputRequest> CopyOutputRequest::CreateRelayRequest(
 }
 
 CopyOutputRequest::CopyOutputRequest()
-    : source_(nullptr),
-      force_bitmap_result_(false),
+    : force_bitmap_result_(false),
       has_area_(false),
       has_texture_mailbox_(false) {}
 
 CopyOutputRequest::CopyOutputRequest(
     bool force_bitmap_result,
     const CopyOutputRequestCallback& result_callback)
-    : source_(nullptr),
-      force_bitmap_result_(force_bitmap_result),
+    : force_bitmap_result_(force_bitmap_result),
       has_area_(false),
       has_texture_mailbox_(false),
       result_callback_(result_callback) {