Bug 902426 - Do not allow duplicate entries in the CRC2D demotion list r=mattwoodrow

--HG--
extra : rebase_source : c7c344f2b88065f46c06c99fa0bf868367b6b9ab

content/canvas/src/CanvasRenderingContext2D.cpp

@@ -797,6 +797,10 @@ CanvasRenderingContext2D::DemoteOldestContextIfNecessary()
 void
 CanvasRenderingContext2D::AddDemotableContext(CanvasRenderingContext2D* context)
 {
+  std::vector<CanvasRenderingContext2D*>::iterator iter = std::find(DemotableContexts().begin(), DemotableContexts().end(), context);
+  if (iter != DemotableContexts().end())
+    return;
+
   DemotableContexts().push_back(context);
 }