Bug 1631476 - Don't let CanvasRenderingContext2D::EnsureTarget lead to frame capture. r=nical

Differential Revision: https://phabricator.services.mozilla.com/D93066

dom/canvas/CanvasRenderingContext2D.cpp

@@ -1316,9 +1316,12 @@ bool CanvasRenderingContext2D::EnsureTarget(const gfx::Rect* aCoveredRect,
   if (mCanvasElement) {
     mCanvasElement->InvalidateCanvas();
   }
+  // EnsureTarget hasn't drawn anything. Preserve mIsCapturedFrameInvalid.
+  bool capturedFrameInvalid = mIsCapturedFrameInvalid;
   // Calling Redraw() tells our invalidation machinery that the entire
   // canvas is already invalid, which can speed up future drawing.
   Redraw();
+  mIsCapturedFrameInvalid = capturedFrameInvalid;
 
   return true;
 }