storage: maintain Lookup size correctly

src/browser/tests/storage.html

@@ -88,3 +88,12 @@
   localStorage.clear();
   testing.expectEqual(0, localStorage.length)
 </script>
+
+<script id="localstorage_limits">
+  localStorage.clear();
+  for (i = 0; i < 5; i++) {
+    const v =  "v".repeat(1024 * 1024);
+    localStorage.setItem(v, v);
+  }
+  testing.expectError("QuotaExceededError", () => localStorage.setItem("last", "v"));
+</script>

src/browser/webapi/storage/storage.zig

@@ -86,11 +86,15 @@ pub const Lookup = struct {
 
     pub fn removeItem(self: *Lookup, key_: ?[]const u8) void {
         const k = key_ orelse return;
+        if (self._data.get(k)) |value| {
+            self._size -= value.len;
             _ = self._data.remove(k);
         }
+    }
 
     pub fn clear(self: *Lookup) void {
         self._data.clearRetainingCapacity();
+        self._size = 0;
     }
 
     pub fn key(self: *const Lookup, index: u32) ?[]const u8 {