[heap-snapshot] Fix context header size

Different contexts have different header sizes now.

The function ScopeInfo::ContextHeaderLength should be used instead of
Context::MIN_CONTEXT_SLOTS.

Bug: chromium:1021884
Change-Id: Ia9ea321cfb19d2a2ae0bc12c7db621f7d35b3b51
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2036074
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66103}

src/profiler/heap-snapshot-generator.cc

@@ -978,7 +978,7 @@ void V8HeapExplorer::ExtractContextReferences(HeapEntry* entry,
     int context_locals = scope_info.ContextLocalCount();
     for (int i = 0; i < context_locals; ++i) {
       String local_name = scope_info.ContextLocalName(i);
-      int idx = Context::MIN_CONTEXT_SLOTS + i;
+      int idx = scope_info.ContextHeaderLength() + i;
       SetContextReference(entry, local_name, context.get(idx),
                           Context::OffsetOfElementAt(idx));
     }