Revert of Remove about:srcdoc url conversion.

content/browser/frame_host/render_frame_host_manager.cc

Index: content/browser/frame_host/render_frame_host_manager.cc
diff --git a/content/browser/frame_host/render_frame_host_manager.cc b/content/browser/frame_host/render_frame_host_manager.cc
index 59cbe436c80a945cff6f16a1e98cea75ad0b679d..380f6871749da153635007161aed49dc9fd1a3d5 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -1406,13 +1406,12 @@
                                   SiteInstanceRelation::UNRELATED);
   }
 
-  // Use the source SiteInstance in case of data URLs, about:srcdoc pages and
-  // about:blank pages because the content is then controlled and/or scriptable
-  // by the source SiteInstance.
+  // Use the source SiteInstance in case of data URLs or about:blank pages,
+  // because the content is then controlled and/or scriptable by the source
+  // SiteInstance.
   GURL about_blank(url::kAboutBlankURL);
-  GURL about_srcdoc(content::kAboutSrcDocURL);
-  if (source_instance && (dest_url == about_srcdoc || dest_url == about_blank ||
-                          dest_url.scheme() == url::kDataScheme)) {
+  if (source_instance &&
+      (dest_url == about_blank || dest_url.scheme() == url::kDataScheme)) {
     return SiteInstanceDescriptor(source_instance);
   }