When autoplay of muted videos is disabled, record the reason why.

third_party/WebKit/Source/core/html/AutoplayUmaHelper.h

Index: third_party/WebKit/Source/core/html/AutoplayUmaHelper.h
diff --git a/third_party/WebKit/Source/core/html/AutoplayUmaHelper.h b/third_party/WebKit/Source/core/html/AutoplayUmaHelper.h
index edfca0b8d490f2fa15796a60997b9947d360f61d..cb991a4de50009a222e3464eb077533e01fc4d26 100644
--- a/third_party/WebKit/Source/core/html/AutoplayUmaHelper.h
+++ b/third_party/WebKit/Source/core/html/AutoplayUmaHelper.h
@@ -27,6 +27,15 @@ enum class AutoplayUnmuteActionStatus {
     NumberOfStatus = 2,
 };
 
+// These values are used for histograms. Do not reorder.
+enum AutoplayBlockedReason {
+    AutoplayBlockedReasonDataSaver = 0,
+    AutoplayBlockedReasonSetting = 1,
+    AutoplayBlockedReasonDataSaverAndSetting = 2,
+    // Keey at the end.
+    AutoplayBlockedReasonMax = 3
+};
+
 class Document;
 class ElementVisibilityObserver;
 class HTMLMediaElement;