webrtc-internals: dont repaint invisible graphs

avoids repainting stats graphs that are not visible.
Visibility is determined as described in
  https://stackoverflow.com/a/21696585/4352772

BUG=678292

Change-Id: Ib7ac623f8d961359e4915b3f5580734fb1328bec
Reviewed-on: https://chromium-review.googlesource.com/602270Reviewed-by: Tommi <tommi@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543372}

content/browser/resources/media/timeline_graph_view.js

@@ -149,9 +149,13 @@ var TimelineGraphView = (function() {
     },
 
     /**
-     * Draws the graph on |canvas_|.
+     * Draws the graph on |canvas_| when visible.
      */
     repaint: function() {
+      if (this.canvas_.offsetParent === null) {
+        return; // do not repaint graphs that are not visible.
+      }
+
       this.repaintTimerRunning_ = false;
 
       var width = this.canvas_.width;