Revert "Fix update external string call in morph string test."

This reverts commit 6b4ba741.

Reason for revert: Blocks roll, see bug.

Bug: v8:7944

Original change's description:
> Fix update external string call in morph string test.
> 
> Bug: chromium:845409
> Change-Id: Ic9b1fa0f669c8b9a82ed91d3f348dd070fc9267f
> Reviewed-on: https://chromium-review.googlesource.com/1135247
> Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54419}

TBR=mlippautz@chromium.org,rfbpb@google.com

Change-Id: I42121de37913bbe858b9a7c6a9dabf86090c493b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:845409
Reviewed-on: https://chromium-review.googlesource.com/1136311Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54427}

test/cctest/test-api.cc

@@ -15654,7 +15654,9 @@ static void MorphAString(i::String* string,
     string->set_map(roots.external_string_map());
     i::ExternalTwoByteString* morphed =
          i::ExternalTwoByteString::cast(string);
-    CcTest::heap()->UpdateExternalString(morphed, string->length(), 0);
+    CcTest::heap()->UpdateExternalString(morphed, morphed->resource()->length(),
+                                         0);
+
     morphed->SetResource(uc16_resource);
   } else {
     // Check old map is not internalized or long.
@@ -15662,7 +15664,8 @@ static void MorphAString(i::String* string,
     // Morph external string to be one-byte string.
     string->set_map(roots.external_one_byte_string_map());
     i::ExternalOneByteString* morphed = i::ExternalOneByteString::cast(string);
-    CcTest::heap()->UpdateExternalString(morphed, string->length(), 0);
+    CcTest::heap()->UpdateExternalString(morphed,
+                                         morphed->resource()->length() * 2, 0);
     morphed->SetResource(one_byte_resource);
   }
 }