Return MaybeHandle from NewExternalStringFrom*.

test/cctest/test-strings.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1109 matching lines @@
 };
 
 
 TEST(SliceFromExternal) {
   FLAG_string_slices = true;
   CcTest::InitializeVM();
   Factory* factory = CcTest::i_isolate()->factory();
   v8::HandleScope scope(CcTest::isolate());
   AsciiVectorResource resource(
       i::Vector<const char>("abcdefghijklmnopqrstuvwxyz", 26));
-  Handle<String> string = factory->NewExternalStringFromAscii(&resource);
+  Handle<String> string =
+      factory->NewExternalStringFromAscii(&resource).ToHandleChecked();
   CHECK(string->IsExternalString());
   Handle<String> slice = factory->NewSubString(string, 1, 25);
   CHECK(slice->IsSlicedString());
   CHECK(string->IsExternalString());
   CHECK_EQ(SlicedString::cast(*slice)->parent(), *string);
   CHECK(SlicedString::cast(*slice)->parent()->IsExternalString());
   CHECK(slice->IsFlat());
 }
 
 
@@ skipping 263 matching lines @@
     dummy.Dispose();                                                           \
   }
 
 INVALID_STRING_TEST(NewStringFromAscii, char)
 INVALID_STRING_TEST(NewStringFromUtf8, char)
 INVALID_STRING_TEST(NewStringFromOneByte, uint8_t)
 INVALID_STRING_TEST(InternalizeOneByteString, uint8_t)
 INVALID_STRING_TEST(InternalizeUtf8String, char)
 
 #undef INVALID_STRING_TEST