[compiler] Temporarily change ContextRef back to kSerialized

This can be reverted once TryMakeRef checks the heap predicate.
I'm not reverting the previous CL because newer changes already depend
on it.

Tbr: jgruber@chromium.org
Bug: v8:11765, v8:7790
Change-Id: Iacc6a78a70fe6f40c9421258889c2175fb400b04
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2891579Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74531}

src/compiler/heap-refs.h

@@ -117,7 +117,7 @@ enum class RefSerializationKind {
   V(CallHandlerInfo, RefSerializationKind::kNeverSerialized)              \
   V(Cell, RefSerializationKind::kNeverSerialized)                         \
   V(Code, RefSerializationKind::kNeverSerialized)                         \
-  V(Context, RefSerializationKind::kNeverSerialized)                      \
+  V(Context, RefSerializationKind::kSerialized)                           \
   V(DescriptorArray, RefSerializationKind::kNeverSerialized)              \
   V(FeedbackCell, RefSerializationKind::kNeverSerialized)                 \
   V(FeedbackVector, RefSerializationKind::kNeverSerialized)               \