fix: only bind ElectronAutofillDriver on primary main frames

The `mojom::ElectronAutofillDriver` associated interface was being
registered for every `RenderFrameHost`, and `AutofillDriverFactory`
would create a driver for any live frame. In practice the native
autofill popup is only exercised from `<datalist>` / `<select>`
elements on the primary main frame — the autofill spec even notes
that `webContents.sendInputEvent` cannot route keyboard input to an
OOPIF, so subframes have no working path to this UI.

Scope the interface to where it is actually used:

* Gate the associated registry entry in
  `ElectronBrowserClient::RegisterAssociatedInterfaceBindersForRenderFrameHost`
  on `IsInPrimaryMainFrame()`, mirroring how `ElectronApiIPC` is
  already scoped in the same function.
* Return early from `AutofillDriverFactory::DriverForFrame` for any
  non-primary-main-frame RFH so the factory never stores a driver
  for an unsupported frame.
* Re-check `IsInPrimaryMainFrame()` inside
  `AutofillDriver::ShowAutofillPopup` as defense in depth for
  messages that race with a navigation.

shell/browser/electron_autofill_driver.cc

@@ -45,6 +45,9 @@ void AutofillDriver::ShowAutofillPopup(
   if (!owner_window)
     return;
 
+  if (!render_frame_host_->IsInPrimaryMainFrame())
+    return;
+
   // |bounds| is supplied by the renderer in the calling frame's RenderWidget
   // coordinate space. Convert to the root view's space and clamp to the
   // calling frame's visible viewport so a (potentially compromised) subframe

shell/browser/electron_autofill_driver_factory.cc

@@ -61,6 +61,8 @@ void AutofillDriverFactory::DidFinishNavigation(
 
 AutofillDriver* AutofillDriverFactory::DriverForFrame(
     content::RenderFrameHost* render_frame_host) {
+  if (!render_frame_host->IsInPrimaryMainFrame())
+    return nullptr;
   auto insertion_result = driver_map_.emplace(render_frame_host, nullptr);
   std::unique_ptr<AutofillDriver>& driver = insertion_result.first->second;
   bool insertion_happened = insertion_result.second;

shell/browser/electron_browser_client.cc

@@ -1541,15 +1541,17 @@ void ElectronBrowserClient::
           },
           &render_frame_host));
 
-  associated_registry.AddInterface<mojom::ElectronAutofillDriver>(
-      base::BindRepeating(
-          [](content::RenderFrameHost* render_frame_host,
-             mojo::PendingAssociatedReceiver<mojom::ElectronAutofillDriver>
-                 receiver) {
-            AutofillDriverFactory::BindAutofillDriver(std::move(receiver),
-                                                      render_frame_host);
-          },
-          &render_frame_host));
+  if (render_frame_host.IsInPrimaryMainFrame()) {
+    associated_registry.AddInterface<mojom::ElectronAutofillDriver>(
+        base::BindRepeating(
+            [](content::RenderFrameHost* render_frame_host,
+               mojo::PendingAssociatedReceiver<mojom::ElectronAutofillDriver>
+                   receiver) {
+              AutofillDriverFactory::BindAutofillDriver(std::move(receiver),
+                                                        render_frame_host);
+            },
+            &render_frame_host));
+  }
 #if BUILDFLAG(ENABLE_PLUGINS)
   associated_registry.AddInterface<mojom::ElectronPluginInfoHost>(
       base::BindRepeating(