Introduce API to trigger exception on JS execution.

src/assert-scope.h

Index: src/assert-scope.h
diff --git a/src/assert-scope.h b/src/assert-scope.h
index 4357056404ebf62a93d7beea8ca9baa7b77d98a6..428e6d007ef8d5a9a5d03ba9484b402a16674bec 100644
--- a/src/assert-scope.h
+++ b/src/assert-scope.h
@@ -49,6 +49,7 @@ enum PerThreadAssertType {
 
 enum PerIsolateAssertType {
   JAVASCRIPT_EXECUTION_ASSERT,
+  JAVASCRIPT_EXECUTION_THROWS,
   ALLOCATION_FAILURE_ASSERT
 };
 
@@ -251,6 +252,14 @@ typedef PerIsolateAssertScope<JAVASCRIPT_EXECUTION_ASSERT, false>
 typedef PerIsolateAssertScope<JAVASCRIPT_EXECUTION_ASSERT, true>
     AllowJavascriptExecution;
 
+// Scope in which javascript execution leads to exception being thrown.
+typedef PerIsolateAssertScope<JAVASCRIPT_EXECUTION_THROWS, false>
+    ThrowOnJavascriptExecution;
+
+// Scope to introduce an exception to ThrowOnJavascriptExecution.
+typedef PerIsolateAssertScope<JAVASCRIPT_EXECUTION_THROWS, true>
+    NoThrowOnJavascriptExecution;
+
 // Scope to document where we do not expect an allocation failure.
 typedef PerIsolateAssertScopeDebugOnly<ALLOCATION_FAILURE_ASSERT, false>
     DisallowAllocationFailure;