Add a |scope| argument to TRACE_EVENT_INSTANT* and require its presence.

cc/scrollbar_layer.cc

Index: cc/scrollbar_layer.cc
diff --git a/cc/scrollbar_layer.cc b/cc/scrollbar_layer.cc
index 6fad23cb11481dd7b64811a269b10d5fe873779f..9d44a8fa35a54d2763af69362af585cdbb7fe155 100644
--- a/cc/scrollbar_layer.cc
+++ b/cc/scrollbar_layer.cc
@@ -268,7 +268,8 @@ void ScrollbarLayer::updatePart(CachingBitmapContentLayerUpdater* painter, Layer
     gfx::Rect paintedOpaqueRect;
     painter->prepareToUpdate(rect, rect.size(), contentsScaleX(), contentsScaleY(), paintedOpaqueRect, stats);
     if (!painter->pixelsDidChange() && resource->texture()->haveBackingTexture()) {
-        TRACE_EVENT_INSTANT0("cc","ScrollbarLayer::updatePart no texture upload needed");
+        TRACE_EVENT_INSTANT0("cc", "ScrollbarLayer::updatePart no texture upload needed",
+                             TRACE_EVENT_SCOPE_THREAD);
         return;
     }