[string] DCHECK instead of test that ThinString's actuals have same length

ThinStrings always forward to internalized strings that have the same
character contents and thus the same length.

Change-Id: I5929d266f96b23029f4786baf993a431cf4ad38d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3541522Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79582}

src/objects/string-table.cc

@@ -719,9 +719,8 @@ Address StringTable::TryStringToIndexOrLookupExisting(Isolate* isolate,
   }
   if (source.IsThinString()) {
     source = ThinString::cast(source).actual();
-    if (string.length() == source.length()) {
-      return source.ptr();
-    }
+    DCHECK_EQ(string.length(), source.length());
+    return source.ptr();
   }
 
   if (source.IsOneByteRepresentation()) {