Move code writing pycairo images to a global function

This will allow follow-up CLs to reuse this code for generating other
images using pycairo.

Bug: 364549423
Change-Id: I340bea1153a82145f06c859e44fc4828d9a38d8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5891403
Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1361438}

html/canvas/tools/gentestutilsunion.py

@@ -332,6 +332,20 @@ def _preprocess_code(jinja_env: jinja2.Environment, code: str,
     return code
 
 
+def _write_cairo_images(pycairo_code: str, output_files: _OutputPaths,
+                        canvas_types: FrozenSet[_CanvasType]) -> None:
+    """Creates a png from pycairo code, for the specified canvas types."""
+    if _CanvasType.HTML_CANVAS in canvas_types:
+        full_code = (f'{pycairo_code}\n'
+                     f'surface.write_to_png("{output_files.element}")\n')
+        eval(compile(full_code, '<string>', 'exec'), {'cairo': cairo})
+
+    if {_CanvasType.OFFSCREEN_CANVAS, _CanvasType.WORKER} & canvas_types:
+        full_code = (f'{pycairo_code}\n'
+                     f'surface.write_to_png("{output_files.offscreen}")\n')
+        eval(compile(full_code, '<string>', 'exec'), {'cairo': cairo})
+
+
 class _Variant():
 
     def __init__(self, params: _MutableTestParams) -> None:
@@ -474,7 +488,6 @@ def finalize_params(self, jinja_env: jinja2.Environment,
     def generate_expected_image(self, output_dirs: _OutputPaths) -> None:
         """Creates a reference image using Cairo and save filename in params."""
         expected = self.params['expected']
-        name = self.params['name']
 
         if expected == 'green':
             self._params['expected_img'] = '/images/green-100x50.png'
@@ -487,23 +500,10 @@ def generate_expected_image(self, output_dirs: _OutputPaths) -> None:
             r'surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, \1, \2)'
             r'\ncr = cairo.Context(surface)', expected)
 
-        output_paths = output_dirs.sub_path(name)
-        if _CanvasType.HTML_CANVAS in self.params['canvas_types']:
-            expected_canvas = (
-                f'{expected}\n'
-                f'surface.write_to_png("{output_paths.element}.png")\n')
-            eval(compile(expected_canvas, f'<test {name}>', 'exec'), {},
-                 {'cairo': cairo})
-
-        if {_CanvasType.OFFSCREEN_CANVAS, _CanvasType.WORKER
-            } & self.params['canvas_types']:
-            expected_offscreen = (
-                f'{expected}\n'
-                f'surface.write_to_png("{output_paths.offscreen}.png")\n')
-            eval(compile(expected_offscreen, f'<test {name}>', 'exec'), {},
-                 {'cairo': cairo})
-
-        self._params['expected_img'] = f'{name}.png'
+        img_filename = f'{self.params["name"]}.png'
+        _write_cairo_images(expected, output_dirs.sub_path(img_filename),
+                            self.params['canvas_types'])
+        self._params['expected_img'] = img_filename
 
 
 class _VariantGrid: