Only query OfflinePageModel in experiment arm

Undos change in https://chromium-review.googlesource.com/c/chromium/src/+/1748081
which was used for debugging.

Bug: 969163
Change-Id: I2008c4f467947f19143bf070b9263f03cc215c4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807334Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696923}

chrome/browser/previews/previews_offline_helper.cc

@@ -146,7 +146,9 @@ PreviewsOfflineHelper::PreviewsOfflineHelper(
       offline_pages::OfflinePageModelFactory::GetForBrowserContext(
           browser_context);
 
-  if (offline_page_model_) {
+  if (offline_page_model_ &&
+      base::FeatureList::IsEnabled(
+          previews::features::kOfflinePreviewsFalsePositivePrevention)) {
     offline_page_model_->AddObserver(this);
     // Schedule a low priority task with a slight delay to ensure that the
     // expensive DB query doesn't occur during startup or during other user