[TFLite] Make TFLite lab experiment observer behind a switch.

This was behind a build flag but we also need it behind a switch
so it isn't running in production for no reason.

Bug: 1165517
Change-Id: I026e8a95a053efd000062c44b1ce7ff5e0aae845
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2638413Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844936}

chrome/browser/ui/tab_helpers.cc

@@ -71,6 +71,7 @@
 #include "chrome/browser/sync/sessions/sync_sessions_web_contents_router_factory.h"
 #include "chrome/browser/sync/sync_encryption_keys_tab_helper.h"
 #include "chrome/browser/tab_contents/navigation_metrics_recorder.h"
+#include "chrome/browser/tflite_experiment/tflite_experiment_switches.h"
 #include "chrome/browser/translate/chrome_translate_client.h"
 #include "chrome/browser/ui/autofill/chrome_autofill_client.h"
 #include "chrome/browser/ui/find_bar/find_bar_state.h"
@@ -275,7 +276,8 @@ void TabHelpers::AttachTabHelpers(WebContents* web_contents) {
       web_contents);
 
 #if BUILDFLAG(BUILD_WITH_TFLITE_LIB)
-  TFLiteExperimentObserver::CreateForWebContents(web_contents);
+  if (tflite_experiment::switches::GetTFLiteModelPath())
+    TFLiteExperimentObserver::CreateForWebContents(web_contents);
 #endif
 
   if (MediaEngagementService::IsEnabled())