Adds a first integration test for the Recent Tabs feature.

components/offline_pages/core/offline_page_feature.cc

Index: components/offline_pages/core/offline_page_feature.cc
diff --git a/components/offline_pages/core/offline_page_feature.cc b/components/offline_pages/core/offline_page_feature.cc
index ef2b4700ca49aa982799f4ad83b8f395822f47c7..9f283129f19998394c55e78a14e01f77ed38b763 100644
--- a/components/offline_pages/core/offline_page_feature.cc
+++ b/components/offline_pages/core/offline_page_feature.cc
@@ -6,8 +6,19 @@
 
 #include <string>
 
+#include "base/command_line.h"
 #include "base/feature_list.h"
 
+namespace switches {
+
+// This flag significantly shortens the delay between WebContentsObserver events
+// and SnapshotController's StartSnapshot calls. The purpose is to speed up
+// integration tests.
+const char kOfflinePagesUseTestingSnapshotDelay[] =
+    "short-offline-page-snapshot-delay-for-test";
+
+}  // namespace switches
+
 namespace offline_pages {
 
 const base::Feature kOfflineBookmarksFeature{"OfflineBookmarks",
@@ -68,4 +79,9 @@ bool ShouldUseNewBackgroundLoader() {
   return base::FeatureList::IsEnabled(kNewBackgroundLoaderFeature);
 }
 
+bool ShouldUseTestingSnapshotDelay() {
+  base::CommandLine* cl = base::CommandLine::ForCurrentProcess();
+  return cl->HasSwitch(switches::kOfflinePagesUseTestingSnapshotDelay);
+}
+
 }  // namespace offline_pages