fix: isolate Pepper plugins (#28373)

* fix: isolate Pepper plugins

Following suit with a recent change to the same method in Chromium, we
should also isloate Pepper plugins.

* docs: add more context to comment

* fix: remove unsupported test flag behavior

Co-authored-by: clavin <cwatford@slack-corp.com>

shell/renderer/renderer_client_base.cc

@@ -383,11 +383,8 @@ bool RendererClientBase::IsPluginHandledExternally(
 
 bool RendererClientBase::IsOriginIsolatedPepperPlugin(
     const base::FilePath& plugin_path) {
-#if BUILDFLAG(ENABLE_PDF_VIEWER)
-  return plugin_path.value() == kPdfPluginPath;
-#else
-  return false;
-#endif
+  // Isolate all Pepper plugins, including the PDF plugin.
+  return true;
 }
 
 std::unique_ptr<blink::WebPrescientNetworking>