Add @requires_offscreen_canvas annotation (#25428)

Add @requires_offscreen_canvas annotation to
browser.test_emscripten_animate_canvas_element_size_offscreen. This is
over-conservative, but keeping test code compact for now.

Author: juj

test/test_browser.py

@@ -4647,6 +4647,7 @@ def test_pthread_run_script(self, args):
 
   # Tests emscripten_set_canvas_element_size() and OffscreenCanvas functionality in different build configurations.
   @requires_graphics_hardware
+  @requires_offscreen_canvas
   @parameterized({
     '': ([], True),
     'offscreen': (['-sOFFSCREENCANVAS_SUPPORT'], True),