Bug 1719375 - Consider documents used-as-an-image always active. r=tnikkel

These are ticked when painted, so it is fine and it's effectively what
happened before bug 1717983 (because we didn't get into this condition:
https://hg.mozilla.org/mozilla-central/rev/cb704553fc64#l3.73).

Differential Revision: https://phabricator.services.mozilla.com/D119314

layout/base/PresShell.cpp

@@ -10788,6 +10788,14 @@ bool PresShell::ShouldBeActive() const {
            mIsActive));
 
   Document* doc = mDocument;
+
+  if (doc->IsBeingUsedAsImage()) {
+    // Documents used as an image can remain active. They do not tick their
+    // refresh driver if not painted, and they can't run script or such so they
+    // can't really observe much else.
+    return true;
+  }
+
   if (Document* displayDoc = doc->GetDisplayDocument()) {
     // Ok, we're an external resource document -- we need to use our display
     // document's docshell to determine "IsActive" status, since we lack