Disallow certain blocking DOM calls during microtask execution.

third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp

Index: third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
diff --git a/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp b/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
index 82cec3ae108b8db111ea29900d13d2138e06ad25..23687c66cd72f9cdc7ed7f221b6b4a1ee6af8d53 100644
--- a/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
+++ b/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
@@ -633,6 +633,15 @@ bool XMLHttpRequest::initSend(ExceptionState& exceptionState)
         return false;
     }
 
+    if (!m_async && exceptionState.isolate() && v8::MicrotasksScope::IsRunningMicrotasks(exceptionState.isolate())) {
+        Deprecation::countDeprecation(getExecutionContext(), UseCounter::During_Microtask_SyncXHR);
+        if (RuntimeEnabledFeatures::disableBlockingMethodsDuringMicrotasksEnabled()) {
+            exceptionState.throwDOMException(InvalidAccessError, "Cannot send() synchronous requests during microtask execution.");
+            return false;
+        }
+    }
+
+
     m_error = false;
     return true;
 }