Have sendBeacon throw for Blobs with a type that is not CORS-safelisted.

third_party/WebKit/Source/modules/beacon/NavigatorBeacon.cpp

Index: third_party/WebKit/Source/modules/beacon/NavigatorBeacon.cpp
diff --git a/third_party/WebKit/Source/modules/beacon/NavigatorBeacon.cpp b/third_party/WebKit/Source/modules/beacon/NavigatorBeacon.cpp
index 972a63f05df0b64f2e2cf345dc12042e417771af..9bb389420f8e1a4ae4ded1c2712ee39cc98608d0 100644
--- a/third_party/WebKit/Source/modules/beacon/NavigatorBeacon.cpp
+++ b/third_party/WebKit/Source/modules/beacon/NavigatorBeacon.cpp
@@ -127,9 +127,9 @@ bool NavigatorBeacon::SendBeaconImpl(
       if (RuntimeEnabledFeatures::
               sendBeaconThrowForBlobWithNonSimpleTypeEnabled()) {
         exception_state.ThrowSecurityError(
-            "sendBeacon() with a Blob whose type is not CORS-safelisted MIME "
-            "type is disallowed experimentally. See http://crbug.com/490015 "
-            "for details.");
+            "sendBeacon() with a Blob whose type is not any of the "
+            "CORS-safelisted values for the Content-Type request header is "
+            "disabled temporarily. See http://crbug.com/490015 for details.");
         return false;
       }
     }