Bug 1493225, part 3 - Cancel content JS when navigating through history to prevent hangs r=smaug

This patch adds an ID to ensure that we avoid canceling content JS if the next
page already started loading by the time we're ready to try canceling the JS.

Differential Revision: https://phabricator.services.mozilla.com/D25164

docshell/base/nsDocShell.cpp

@@ -667,6 +667,16 @@ nsDocShell::GetInterface(const nsIID& aIID, void** aSink) {
   return *aSink ? NS_OK : NS_NOINTERFACE;
 }
 
+NS_IMETHODIMP
+nsDocShell::SetCancelContentJSEpoch(int32_t aEpoch) {
+  // Note: this gets called fairly early (before a pageload actually starts).
+  // We could probably defer this even longer.
+  nsCOMPtr<nsIBrowserChild> browserChild = GetBrowserChild();
+  static_cast<BrowserChild*>(browserChild.get())
+      ->SetCancelContentJSEpoch(aEpoch);
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDocShell::LoadURI(nsDocShellLoadState* aLoadState) {
   MOZ_ASSERT(aLoadState, "Must have a valid load state!");