Get signals working in the EXTRA_DATA section of MHTML

chrome/browser/android/offline_pages/prerendering_offliner.cc

Index: chrome/browser/android/offline_pages/prerendering_offliner.cc
diff --git a/chrome/browser/android/offline_pages/prerendering_offliner.cc b/chrome/browser/android/offline_pages/prerendering_offliner.cc
index a6564ad078d662af7b164c009e039d8dd89aa383..c25571c11b80ce828fc1007e991b10dc70e4cff2 100644
--- a/chrome/browser/android/offline_pages/prerendering_offliner.cc
+++ b/chrome/browser/android/offline_pages/prerendering_offliner.cc
@@ -16,8 +16,13 @@
 #include "components/offline_pages/core/downloads/download_ui_adapter.h"
 #include "components/offline_pages/core/offline_page_model.h"
 #include "content/public/browser/browser_context.h"
+#include "content/public/browser/mhtml_extra_parts.h"
 #include "content/public/browser/web_contents.h"
 
+namespace {
+const char kContentType[] = "text/plain";
+}  // namespace
+
 namespace offline_pages {
 
 PrerenderingOffliner::PrerenderingOffliner(
@@ -83,6 +88,21 @@ void PrerenderingOffliner::OnLoadPageDone(
     else if (save_page_params.url != request.url())
       save_page_params.original_url = request.url();
 
+#ifndef NDEBUG
+    // Stash loading signals for writing when we write out the MHTML.
+    std::string body = SerializeLoadingSignalData();
+    std::string content_type = kContentType;
+    std::string content_location = "cid:signal-data-" +
+                                   std::to_string(request.request_id()) +
+                                   "@mhtml.blink";
+
+    content::MHTMLExtraParts* extra_parts =
+        content::MHTMLExtraParts::FromWebContents(web_contents);
+    DCHECK(extra_parts);
+
+    extra_parts->AddExtraMHTMLPart(content_type, content_location, body);
+#endif
+
     SavePage(save_page_params, std::move(archiver),
              base::Bind(&PrerenderingOffliner::OnSavePageDone,
                         weak_ptr_factory_.GetWeakPtr(), request));
@@ -94,6 +114,19 @@ void PrerenderingOffliner::OnLoadPageDone(
   }
 }
 
+std::string PrerenderingOffliner::SerializeLoadingSignalData() {
+  // Write the signal data into a single string.
+  std::string signal_string;
+  const std::vector<std::string>& signals = loader_->GetLoadingSignalData();
+
+  // TODO(petewil): Convert this to JSON, use json_writer.h
+  for (std::string signal : signals) {
+    signal_string += signal;
+    signal_string += "\r\n";
+  }
+  return signal_string;
+}
+
 void PrerenderingOffliner::OnSavePageDone(
     const SavePageRequest& request,
     SavePageResult save_result,