canvas: prevent unwrap on offscreen canvas (#37460)

Remove all unwrap usage on offscreen canvas to prevent panic.

Testing:
`tests/wpt/tests/html/canvas/offscreen/manual/the-offscreen-canvas/offscreencanvas.transfercontrol.to.offscreen-crash.html`
Fixes: #37415

---------

Signed-off-by: Wu Yu Wei <yuweiwu@pm.me>
Signed-off-by: Yu Wei Wu <yuweiwu@YunoMacBook-Air.local>
Co-authored-by: Yu Wei Wu <yuweiwu@YunoMacBook-Air.local>

components/script/dom/canvasrenderingcontext2d.rs

@@ -140,8 +140,8 @@ impl CanvasContext for CanvasRenderingContext2D {
         self.canvas_state.get_canvas_id()
     }
 
-    fn canvas(&self) -> HTMLCanvasElementOrOffscreenCanvas {
-        self.canvas.clone()
+    fn canvas(&self) -> Option<HTMLCanvasElementOrOffscreenCanvas> {
+        Some(self.canvas.clone())
     }
 
     fn update_rendering(&self) {

components/script/dom/offscreencanvasrenderingcontext2d.rs

@@ -75,7 +75,7 @@ impl CanvasContext for OffscreenCanvasRenderingContext2D {
         self.context.context_id()
     }
 
-    fn canvas(&self) -> HTMLCanvasElementOrOffscreenCanvas {
+    fn canvas(&self) -> Option<HTMLCanvasElementOrOffscreenCanvas> {
         self.context.canvas()
     }
 
@@ -98,7 +98,7 @@ impl OffscreenCanvasRenderingContext2DMethods<crate::DomTypeHolder>
     // https://html.spec.whatwg.org/multipage/offscreencontext2d-canvas
     fn Canvas(&self) -> DomRoot<OffscreenCanvas> {
         match self.context.canvas() {
-            HTMLCanvasElementOrOffscreenCanvas::OffscreenCanvas(canvas) => canvas,
+            Some(HTMLCanvasElementOrOffscreenCanvas::OffscreenCanvas(canvas)) => canvas,
             _ => panic!("Should not be called from onscreen canvas"),
         }
     }

components/script/dom/webgl2renderingcontext.rs

@@ -908,7 +908,7 @@ impl CanvasContext for WebGL2RenderingContext {
         self.base.context_id()
     }
 
-    fn canvas(&self) -> HTMLCanvasElementOrOffscreenCanvas {
+    fn canvas(&self) -> Option<HTMLCanvasElementOrOffscreenCanvas> {
         self.base.canvas().clone()
     }
 

components/script/dom/webglrenderingcontext.rs

@@ -1914,8 +1914,8 @@ impl CanvasContext for WebGLRenderingContext {
         self.webgl_sender.context_id()
     }
 
-    fn canvas(&self) -> HTMLCanvasElementOrOffscreenCanvas {
-        self.canvas.clone()
+    fn canvas(&self) -> Option<HTMLCanvasElementOrOffscreenCanvas> {
+        Some(self.canvas.clone())
     }
 
     fn resize(&self) {

components/script/dom/webgpu/gpucanvascontext.rs

@@ -294,8 +294,8 @@ impl CanvasContext for GPUCanvasContext {
         })
     }
 
-    fn canvas(&self) -> HTMLCanvasElementOrOffscreenCanvas {
-        self.canvas.clone()
+    fn canvas(&self) -> Option<HTMLCanvasElementOrOffscreenCanvas> {
+        Some(self.canvas.clone())
     }
 }