Add fonts to VariantGrid params and default Variant params

For fonts to be loaded from the templates for single-file test grids,
they must be visible from the grid as a whole, and they must be
aggregated for cases with different fonts declared in different
variants.

This change adds the `fonts` param to grids using `_param_set`, creates
it as an empty list when initializing a default variant.

In a follow-up CL in the stack, a test is created that requires the
loading of fonts and uses single-file variant grids.

Bug: 341213359
Change-Id: Id6d83d26ab4cfa9a98747d573926fc40a24ab565
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5635554
Reviewed-by: Jean-Philippe Gravel <[email protected]>
Commit-Queue: Andres Ricardo Perez <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1322188}

html/canvas/tools/gentestutilsunion.py

@@ -360,6 +360,7 @@ def create_with_defaults(test: _TestParams) -> '_Variant':
             'variant_name': '',
             'images': [],
             'svgimages': [],
+            'fonts': [],
         }
         params.update(test)
         return _Variant(params)
@@ -639,6 +640,7 @@ def _get_grid_params(self) -> _TestParams:
             'notes': self._unique_param('notes'),
             'images': self._param_set('images'),
             'svgimages': self._param_set('svgimages'),
+            'fonts': self._param_set('fonts'),
         }
         if self.template_type in (_TemplateType.REFERENCE,
                                   _TemplateType.HTML_REFERENCE):