Bug 1555488 - Part 1: Clear WindowProxy cache when flipping processes, r=peterv,mccr8

This is sorta-part of the work for bug 1510760. Currently if you do a process
switch, the window proxy state is completely borked, and a lot of stuff doesn't
work.

This patch aims to make it so that, while not transplanted correctly, methods
generally don't return the wrong WindowProxy object anymore after a process
swap.

Differential Revision: https://phabricator.services.mozilla.com/D33082

docshell/base/nsDocShell.cpp

@@ -5072,9 +5072,11 @@ nsDocShell::Destroy() {
     mSessionHistory = nullptr;
   }
 
-  // This will be skipped in cases where we want to preserve the browsing
-  // context between loads.
-  if (!mSkipBrowsingContextDetachOnDestroy) {
+  // Either `Detach` our BrowsingContext if this window is closing, or prepare
+  // the BrowsingContext for the switch to continue.
+  if (mSkipBrowsingContextDetachOnDestroy) {
+    mBrowsingContext->PrepareForProcessChange();
+  } else {
     mBrowsingContext->Detach();
   }