Bug 1721217 - Part 2: Fix the race between navigations originating in the parent and in the content, r=nika

Add a synced ParentInitiatedNavigationEpoch field to browsing context, which
only gets incremented when we start navigations in the parent process. When a
child process initiates a navigation, it sends the current value of the field
that it sees via DocumentChannelCreationArgs. In the parent process, we can
compare the value of that field with the latest one for the same browsing
context. If the latest value is higher than the one provided by the content
process, it means that in the meantime parent process has started a navigation
so the earlier navigation originating in the content process will be cancelled.

Differential Revision: https://phabricator.services.mozilla.com/D126842

docshell/base/BrowsingContext.cpp

@@ -3182,6 +3182,11 @@ auto BrowsingContext::CanSet(FieldIndex<IDX_CurrentInnerWindowId>,
   return CanSetResult::Allow;
 }
 
+bool BrowsingContext::CanSet(FieldIndex<IDX_ParentInitiatedNavigationEpoch>,
+                             const uint64_t& aValue, ContentParent* aSource) {
+  return XRE_IsParentProcess() && !aSource;
+}
+
 void BrowsingContext::DidSet(FieldIndex<IDX_CurrentInnerWindowId>) {
   RefPtr<WindowContext> prevWindowContext = mCurrentWindowContext.forget();
   mCurrentWindowContext = WindowContext::GetById(GetCurrentInnerWindowId());