Add UseCounters for alert(), confirm() and prompt() usage against site engagement.

third_party/WebKit/Source/core/frame/UseCounter.h

Index: third_party/WebKit/Source/core/frame/UseCounter.h
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.h b/third_party/WebKit/Source/core/frame/UseCounter.h
index 68806e322e061809e429581cf1b1f68ea71d11c6..0be332038f314a6e78c53a0a434fb5fde90e8d38 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.h
+++ b/third_party/WebKit/Source/core/frame/UseCounter.h
@@ -1401,6 +1401,24 @@ class CORE_EXPORT UseCounter {
     NavigatorVibrateEngagementMedium = 1728,
     NavigatorVibrateEngagementHigh = 1729,
     NavigatorVibrateEngagementMax = 1730,
+    AlertEngagementNone = 1731,
+    AlertEngagementMinimal = 1732,
+    AlertEngagementLow = 1733,
+    AlertEngagementMedium = 1734,
+    AlertEngagementHigh = 1735,
+    AlertEngagementMax = 1736,
+    ConfirmEngagementNone = 1737,
+    ConfirmEngagementMinimal = 1738,
+    ConfirmEngagementLow = 1739,
+    ConfirmEngagementMedium = 1740,
+    ConfirmEngagementHigh = 1741,
+    ConfirmEngagementMax = 1742,
+    PromptEngagementNone = 1743,
+    PromptEngagementMinimal = 1744,
+    PromptEngagementLow = 1745,
+    PromptEngagementMedium = 1746,
+    PromptEngagementHigh = 1747,
+    PromptEngagementMax = 1748,
 
     // Add new features immediately above this line. Don't change assigned
     // numbers of any item, and don't reuse removed slots.