Bug 1377131 - Try to trigger collector slices at times which disturb page js less (at least with iframes loaded after the top level page has been loaded), r=mccr8,bz

When triggering an iframe load or starting to parse a document for an iframe, the main thread may often have some time before the new page has been created. Try to trigger CC/GC slice at such point in order to avoid collector later when page is already executing its JS

docshell/base/nsDocShell.cpp

@@ -222,6 +222,7 @@
 #include "mozilla/dom/PerformanceNavigation.h"
 #include "mozilla/dom/ScriptSettings.h"
 #include "mozilla/Encoding.h"
+#include "nsJSEnvironment.h"
 #include "IUrlClassifierUITelemetry.h"
 
 #ifdef MOZ_TOOLKIT_SEARCH
@@ -11704,6 +11705,11 @@ nsDocShell::DoChannelLoad(nsIChannel* aChannel,
   rv = aURILoader->OpenURI(aChannel, openFlags, this);
   NS_ENSURE_SUCCESS(rv, rv);
 
+  // We're about to load a new page and it may take time before necko
+  // gives back any data, so main thread might have a chance to process a
+  // collector slice
+  nsJSContext::MaybeRunNextCollectorSlice(this, JS::gcreason::DOCSHELL);
+
   return NS_OK;
 }