Bug 984845 - BrowserElementParent.getScreenshot should return more accurate screenshots. r=fabrice

dom/browser-element/BrowserElementChildPreload.js

@@ -831,10 +831,16 @@ BrowserElementChild.prototype = {
     canvas.width = canvasWidth;
     canvas.height = canvasHeight;
 
-    var ctx = canvas.getContext("2d", { willReadFrequently: true });
+    let ctx = canvas.getContext("2d", { willReadFrequently: true });
     ctx.scale(scale * devicePixelRatio, scale * devicePixelRatio);
+
+    let flags = ctx.DRAWWINDOW_DRAW_VIEW |
+                ctx.DRAWWINDOW_USE_WIDGET_LAYERS |
+                ctx.DRAWWINDOW_DO_NOT_FLUSH |
+                ctx.DRAWWINDOW_ASYNC_DECODE_IMAGES;
     ctx.drawWindow(content, 0, 0, content.innerWidth, content.innerHeight,
-                   transparent ? "rgba(255,255,255,0)" : "rgb(255,255,255)");
+                   transparent ? "rgba(255,255,255,0)" : "rgb(255,255,255)",
+                   flags);
 
     // Take a JPEG screenshot by default instead of PNG with alpha channel.
     // This requires us to unpremultiply the alpha channel, which