Bug 1837070 - Ensure we flush OffscreenCanvas when the display list is recreated. r=lsalzman

When we recreate the display list, e.g. after tabbing away, we need to
ensure we flush the OffscreenCanvas because our resource handles will be
invalidated. This requires the OffscreenCanvasDisplayHelper to know
which OffscreenCanvas it belongs to, and a worker reference if necessary
to do the dispatching for the flush. Otherwise we would require the
canvas to be updated manually by the web application to repaint.

Differential Revision: https://phabricator.services.mozilla.com/D184324

dom/html/HTMLCanvasElement.cpp

@@ -485,7 +485,7 @@ HTMLCanvasElement::~HTMLCanvasElement() { Destroy(); }
 
 void HTMLCanvasElement::Destroy() {
   if (mOffscreenDisplay) {
-    mOffscreenDisplay->Destroy();
+    mOffscreenDisplay->DestroyElement();
     mOffscreenDisplay = nullptr;
     mImageContainer = nullptr;
   }
@@ -1095,6 +1095,12 @@ void HTMLCanvasElement::SetHeight(uint32_t aHeight, ErrorResult& aRv) {
   SetUnsignedIntAttr(nsGkAtoms::height, aHeight, DEFAULT_CANVAS_HEIGHT, aRv);
 }
 
+void HTMLCanvasElement::FlushOffscreenCanvas() {
+  if (mOffscreenDisplay) {
+    mOffscreenDisplay->FlushForDisplay();
+  }
+}
+
 void HTMLCanvasElement::InvalidateCanvasPlaceholder(uint32_t aWidth,
                                                     uint32_t aHeight) {
   ErrorResult rv;