metrics: Simplify ProgressiveWebMetrics (#35985)

Simply how `ProgressiveWebMetrics` works:

1. Keep only a single struct instead of one in layout and one script
   that both implement the `ProgressiveWebMetrics` trait. Since layout
   and script are the same thread these can now just be a single
   `ProgressiveWebMetrics` struct stored in script.
2. Have the compositor be responsible for informing the Constellation
   (which informs the ScripThread) about paint metrics. This makes
   communication flow one way and removes one dependency between the
   compositor and script (of two).
3. All units tests are moved into the `metrics` crate itself since there
   is only one struct there now.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

python/servo/testing_commands.py

@@ -166,6 +166,7 @@ class MachCommands(CommandBase):
             "hyper_serde",
             "layout_2020",
             "libservo",
+            "metrics",
             "net",
             "net_traits",
             "pixels",