[v8] Add base::feature for Symbols-as-WeakMap-keys

I2S with 3 LGTMs:
https://groups.google.com/a/chromium.org/g/blink-dev/c/E6pDZP_TiBA/m/ZcXLwiz8AAAJ

Bug: v8:12947
Change-Id: Id9f70f79833272df4416dd1ea349a7e53aac7ea5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4005012
Reviewed-by: Camillo Bruni <[email protected]>
Commit-Queue: Shu-yu Guo <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1067545}
diff --git a/gin/gin_features.cc b/gin/gin_features.cc
index 89de137..4068b86 100644
--- a/gin/gin_features.cc
+++ b/gin/gin_features.cc
@@ -150,4 +150,11 @@
 const base::FeatureParam<base::TimeDelta> kV8MemoryReducerStartDelay{
     &kV8DelayMemoryReducer, "delay", base::Seconds(8)};
 
+// JavaScript language features.
+
+// Enables the Symbols-as-WeakMap-keys proposal.
+BASE_FEATURE(kJavaScriptSymbolAsWeakMapKey,
+             "JavaScriptSymbolAsWeakMapKey",
+             base::FEATURE_ENABLED_BY_DEFAULT);
+
 }  // namespace features