Move the writing of the MHTML footer to the browser process.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index 4af9b671725620814b3f5d10384a881f4c68d83a..2e8f6a1bf9f365910cf4b918cc3cbe74679c0e1a 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -5544,14 +5544,8 @@ void RenderFrameImpl::OnSerializeAsMHTML(
     has_some_data |= !mhtml_contents.back().isEmpty();
   }
 
-  // Generate MHTML footer if needed.
-  if (save_status == MhtmlSaveStatus::SUCCESS && params.is_last_frame) {
-    TRACE_EVENT0("page-serialization",
-                 "RenderFrameImpl::OnSerializeAsMHTML footer");
-    mhtml_contents.emplace_back(
-        WebFrameSerializer::generateMHTMLFooter(mhtml_boundary));
-    has_some_data |= !mhtml_contents.back().isEmpty();
-  }
+  // Note: the MHTML footer is written by the browser process, after the last
+  // frame is serialized by a renderer process.
 
   // Note: we assume RenderFrameImpl::OnWriteMHTMLToDiskComplete and the rest of
   // this function will be fast enough to not need to be accounted for in this