[string] Add additional ThinString test

Add a test for the case where SlicedStrings of ThinStrings are looked up
in the string table, testing the path that the original string's length
differs from the actual string's length.

Bug: chromium:1309767
Change-Id: I909c64397bf28ec33c3324d94882fbfe81ac4109
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3549837Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79610}

test/mjsunit/thin-strings.js

@@ -94,3 +94,18 @@ cc2(t);
 cc2(t);
 %OptimizeFunctionOnNextCall(cc2);
 cc2(t);
+
+function string_table_lookup_sliced_thin_string(a, b) {
+  // Make a ConsString.
+  var s = a + b;
+  // Slice a substring out of it.
+  var slice = s.substring(0, 20);
+  // Make the original string thin.
+  var o = {};
+  o[s];
+  // Try to internalize the SlicedString.
+  o[slice];
+}
+
+string_table_lookup_sliced_thin_string(
+    'abcdefghijklmnopqrstuvwxyz', '0123456789');