Adds counter for snapshot decompress time when creating context

BUG=chromium:1257321
TEST=none

Change-Id: I59f34e8b41ba08f5046754c13be8f1df6a335655
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3271389Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77824}

src/logging/counters-definitions.h

@@ -131,8 +131,11 @@ namespace internal {
   /* Time for lazily compiling Wasm functions. */                             \
   HT(wasm_lazy_compile_time, V8.WasmLazyCompileTimeMicroSeconds, 100000000,   \
      MICROSECOND)                                                             \
-  /* Total time to decompress snapshots. */                                   \
-  HT(snapshot_decompress, V8.SnapshotDecompress, 10000000, MICROSECOND)
+  /* Total time to decompress isolate snapshot. */                            \
+  HT(snapshot_decompress, V8.SnapshotDecompress, 10000000, MICROSECOND)       \
+  /* Time to decompress context snapshot. */                                  \
+  HT(context_snapshot_decompress, V8.ContextSnapshotDecompress, 10000000,     \
+     MICROSECOND)
 
 #define NESTED_TIMED_HISTOGRAM_LIST_SLOW(HT)                               \
   /* Total V8 time (including JS and runtime calls, exluding callbacks) */ \

src/snapshot/snapshot.cc

@@ -210,10 +210,20 @@ MaybeHandle<Context> Snapshot::NewContextFromSnapshot(
   bool can_rehash = ExtractRehashability(blob);
   base::Vector<const byte> context_data = SnapshotImpl::ExtractContextData(
       blob, static_cast<uint32_t>(context_index));
-  SnapshotData snapshot_data(MaybeDecompress(context_data));
+  base::Optional<SnapshotData> snapshot_data;
+  {
+#ifdef V8_SNAPSHOT_COMPRESSION
+    base::Optional<NestedTimedHistogramScope> decompress_histogram;
+    if (base::TimeTicks::IsHighResolution()) {
+      decompress_histogram.emplace(
+          isolate->counters()->context_snapshot_decompress());
+    }
+#endif
+    snapshot_data.emplace(MaybeDecompress(context_data));
+  }
 
   MaybeHandle<Context> maybe_result = ContextDeserializer::DeserializeContext(
-      isolate, &snapshot_data, can_rehash, global_proxy,
+      isolate, &(*snapshot_data), can_rehash, global_proxy,
       embedder_fields_deserializer);
 
   Handle<Context> result;