Add UseCounters for the types of mixed content we currently don't block.

Source/core/frame/UseCounter.h

Index: Source/core/frame/UseCounter.h
diff --git a/Source/core/frame/UseCounter.h b/Source/core/frame/UseCounter.h
index c887cb699e429adae53ce6d5d22e431a390dde57..2ed3422a7828a9db51a75dcf027912adafe221f4 100644
--- a/Source/core/frame/UseCounter.h
+++ b/Source/core/frame/UseCounter.h
@@ -421,7 +421,6 @@ public:
         PostMessageFromInsecureToSecure = 420,
         DocumentExitPointerLock = 421,
         DocumentPointerLockElement = 422,
-        MixedContentFont = 423,
         PrefixedCursorZoomIn = 424,
         PrefixedCursorZoomOut = 425,
         CSSCharsetRuleEncoding = 426,
@@ -434,6 +433,11 @@ public:
         FocusInOutEvent = 433,
         MouseEventMovementX = 434,
         MouseEventMovementY = 435,
+        MixedContentTextTrack = 436,
+        MixedContentRaw = 437,
+        MixedContentImage = 438,
+        MixedContentMedia = 439,
+
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.
         // Also, run update_use_counter_feature_enum.py in chromium/src/tools/metrics/histograms/