Protector adds the default prepopulated engine if it was removed.

chrome/browser/protector/histograms.h

Index: chrome/browser/protector/histograms.h
diff --git a/chrome/browser/protector/histograms.h b/chrome/browser/protector/histograms.h
index c27c379eeee274521458edbe83b8468e8bfb38d7..5dc301ecb3e34849cd0ca8282c470da4601fbecc 100644
--- a/chrome/browser/protector/histograms.h
+++ b/chrome/browser/protector/histograms.h
@@ -37,6 +37,15 @@ extern const char kProtectorHistogramNewSearchProvider[];
 extern const char kProtectorHistogramSearchProviderApplied[];
 // Histogram name to report when user keeps previous default search provider.
 extern const char kProtectorHistogramSearchProviderDiscarded[];
+// Histogram name to report the fallback default search provider when the
+// backup value is invalid or doesn't match an existing provider.
+extern const char kProtectorHistogramSearchProviderFallback[];
+// Histogram name to report when the prepopulated default search provider was
+// missing and has been added for fallback.
+extern const char kProtectorHistogramSearchProviderMissing[];
+// Histogram name to report the default search provider restored by Protector
+// before showing user the bubble.
+extern const char kProtectorHistogramSearchProviderRestored[];
 // Histogram name to report when user ignores search provider change.
 extern const char kProtectorHistogramSearchProviderTimeout[];