script: Remove 'pending reflow' concept and some explicit reflows (#34558)

The `pending reflow` concept isn't necessary now that *update the
rendering* is taking care of triggering reflows at the correct time.
`Window::reflow` already avoids reflows if the page is not dirty, so
pending reflows is now just an extraneous check as long as *update the
rendering* runs properly.

This change also removes some explicit reflows, which now wait until the
appropriate moment during *update the rendering*. This should remove
some extra reflows that are not necessary.

Servo needs some way to track that resizing the web view needs to
re-layout due to the initial containing block changing. Move handling
of `Document::needs_paint` to the script thread and use this, expanding
the rustdoc to explain what it is for a bit more clearly.

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

components/script/layout_dom/document.rs

@@ -49,14 +49,6 @@ impl<'ld> ServoLayoutDocument<'ld> {
             .next()
     }
 
-    pub fn needs_paint_from_layout(&self) {
-        self.document.needs_paint_from_layout()
-    }
-
-    pub fn will_paint(&self) {
-        self.document.will_paint()
-    }
-
     pub fn style_shared_lock(&self) -> &StyleSharedRwLock {
         self.document.style_shared_lock()
     }