[ic] Make prototypes fast when storing through keyed store IC.

Toon suggested this as a mitigation to the problem of prototype fast mode switching invalidating prototype chain validity cell, and thus sending keyed store ICs to megamorphic state.

BUG=chromium:723479

Review-Url: https://codereview.chromium.org/2943313002
Cr-Commit-Position: refs/heads/master@{#45993}

src/ic/ic.cc

@@ -2265,6 +2265,10 @@ MaybeHandle<Object> KeyedStoreIC::Store(Handle<Object> object,
     return store_handle;
   }
 
+  if (state() != UNINITIALIZED) {
+    JSObject::MakePrototypesFast(object, kStartAtPrototype, isolate());
+  }
+
   bool use_ic = FLAG_use_ic && !object->IsStringWrapper() &&
                 !object->IsAccessCheckNeeded() && !object->IsJSGlobalProxy();
   if (use_ic && !object->IsSmi()) {