Bug 1846293 - Improve flush handing in Canvas2D. r=dholbert

Don't keep weak pointers before flushing.

Differential Revision: https://phabricator.services.mozilla.com/D184950

dom/canvas/CanvasRenderingContext2D.cpp

@@ -4462,14 +4462,14 @@ TextMetrics* CanvasRenderingContext2D::DrawOrMeasureText(
     const nsAString& aRawText, float aX, float aY,
     const Optional<double>& aMaxWidth, TextDrawOperation aOp,
     ErrorResult& aError) {
-  gfxFontGroup* currentFontStyle = GetCurrentFontStyle();
+  RefPtr<gfxFontGroup> currentFontStyle = GetCurrentFontStyle();
   if (NS_WARN_IF(!currentFontStyle)) {
     aError = NS_ERROR_FAILURE;
     return nullptr;
   }
 
   RefPtr<PresShell> presShell = GetPresShell();
-  Document* document = presShell ? presShell->GetDocument() : nullptr;
+  RefPtr<Document> document = presShell ? presShell->GetDocument() : nullptr;
 
   // replace all the whitespace characters with U+0020 SPACE
   nsAutoString textToDraw(aRawText);