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..7fb38c3c01b398e0dab3ef4641d74b151b6023c2 100644
--- a/chrome/browser/android/offline_pages/prerendering_offliner.cc
+++ b/chrome/browser/android/offline_pages/prerendering_offliner.cc
@@ -15,6 +15,7 @@
 #include "components/offline_pages/core/client_namespace_constants.h"
 #include "components/offline_pages/core/downloads/download_ui_adapter.h"
 #include "components/offline_pages/core/offline_page_model.h"
+#include "content/browser/download/mhtml_extra_data.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/web_contents.h"
 
@@ -83,6 +84,22 @@ void PrerenderingOffliner::OnLoadPageDone(
     else if (save_page_params.url != request.url())
       save_page_params.original_url = request.url();
 
+    // Write the signal data into a single string.
+    const char kContentTypeLine[] = "Content-Type: application/json\r\n";

[Dmitry Titov, 2017/03/16 02:28:20]

this line would normally be at the beginning of the file in anon namespace...

[Pete Williamson, 2017/03/17 23:28:32]

Done.

+    std::string signal_string(kContentTypeLine);
+    const std::vector<std::string>& signals = loader_->GetSignalData();
+
+    // TODO(petewil): Convert this to JSON.
+    for (std::string signal : signals) {

[Dmitry Titov, 2017/03/16 02:28:20]

I'd move this whole block into a separate function,
SerializeLoadCompletenessSignalData or something like this. 

[Pete Williamson, 2017/03/17 23:28:32]

Done.

+      signal_string += signal;
+      signal_string += "\r\n";
+    }
+
+    // Attach the signal data to the web contents.
+    web_contents->SetUserData(content::kMHTMLExtraDataKey,
+                              std::unique_ptr<base::SupportsUserData::Data>(
+                                  new content::MHTMLExtraData(signal_string)));
+
     SavePage(save_page_params, std::move(archiver),
              base::Bind(&PrerenderingOffliner::OnSavePageDone,
                         weak_ptr_factory_.GetWeakPtr(), request));