feat: Corner Smoothing CSS rule (#45185)

* feat: Corner Smoothing

* Patch Blink to add CSS rule and Painting

* Add `system-ui` keyword

* Add `cornerSmoothingCSS` web preference

* Add tests

* Documentation

* fixup! Documentation

* fix: initialize smoothness value

* Use a 1.0 scale factor in tests

* Fix tests for CI

* Fix tests

* fixup! Merge branch 'main' into clavin/corner-smoothing

* Add code docs

* Document `system-ui` keyword values

* Address review comments

* fixup! Address review comments

* Address review comments

* Update patch to address upstream changes

The patch went from 694 lines to 505 lines, which is a 27.2% smaller!

* fixup! Update patch to address upstream changes

shell/renderer/electron_smooth_round_rect.h

@@ -0,0 +1,36 @@
+// Copyright (c) 2024 Salesforce, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#ifndef ELECTRON_SHELL_RENDERER_API_ELECTRON_SMOOTH_ROUND_RECT_H_
+#define ELECTRON_SHELL_RENDERER_API_ELECTRON_SMOOTH_ROUND_RECT_H_
+
+#include "third_party/skia/include/core/SkPath.h"
+
+namespace electron {
+
+// Draws a rectangle that has smooth round corners for a given "smoothness"
+// value between 0.0 and 1.0 (representing 0% to 100%).
+//
+// The smoothness value determines how much edge length can be consumed by each
+// corner, scaling with respect to that corner's radius. The smoothness will
+// dynamically scale back if there is not enough edge length, similar to how
+// the corner radius backs off when there isn't enough edge length.
+//
+// Each corner's radius can be supplied independently. Corner radii are expected
+// to already be balanced (Radius1 + Radius2 <= Length, for each given side).
+//
+// Elliptical corner radii are not currently supported.
+SkPath DrawSmoothRoundRect(float x,
+                           float y,
+                           float width,
+                           float height,
+                           float smoothness,
+                           float top_left_radius,
+                           float top_right_radius,
+                           float bottom_right_radius,
+                           float bottom_left_radius);
+
+}  // namespace electron
+
+#endif  // ELECTRON_SHELL_RENDERER_API_ELECTRON_SMOOTH_ROUND_RECT_H_