Implement scroll handler latency tracking

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 48f4864ef4a14e903aba2665fa85a989f725ea8a..0b7644618c5c24a75ddb629865e3aed5976c07b1 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -18,6 +18,7 @@
 #include "base/strings/string_number_conversions.h"
 #include "cc/animation/animation_registrar.h"
 #include "cc/animation/layer_animation_controller.h"
+#include "cc/base/latency_info_swap_promise.h"
 #include "cc/base/math_util.h"
 #include "cc/debug/devtools_instrumentation.h"
 #include "cc/debug/rendering_stats_instrumentation.h"
@@ -1017,31 +1018,43 @@ void LayerTreeHost::PaintLayerContents(
   in_paint_layer_contents_ = false;
 }
 
-void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
+void LayerTreeHost::ApplyScrollAndScale(ScrollAndScaleSet* info) {
   if (!root_layer_.get())
     return;
 
+  ScopedPtrVector<LatencyInfoSwapPromise>::iterator it =
+      info->swap_promises.begin();
+  for (; it != info->swap_promises.end(); ++it) {
+    scoped_ptr<LatencyInfoSwapPromise> swap_promise(
+        info->swap_promises.take(it));
+    TRACE_EVENT_FLOW_STEP0(
+        "input",
+        "LatencyInfo.Flow",
+        TRACE_ID_DONT_MANGLE(swap_promise->latency_info().trace_id),

[danakj, 2014/07/09 15:50:35]

LGTM but if you were to put a TraceId() accessor on SwapPromise, then you could
keep all this general and not need to use the LatencyInfoSwapPromise type
anywhere in LTH/LTHI which might come in handy if we have another SwapPromise
type in the future.

+        "Main thread scroll update");
+    QueueSwapPromise(swap_promise.PassAs<SwapPromise>());
+  }
+
   gfx::Vector2d inner_viewport_scroll_delta;
   gfx::Vector2d outer_viewport_scroll_delta;
 
-  for (size_t i = 0; i < info.scrolls.size(); ++i) {
-    Layer* layer =
-        LayerTreeHostCommon::FindLayerInSubtree(root_layer_.get(),
-                                                info.scrolls[i].layer_id);
+  for (size_t i = 0; i < info->scrolls.size(); ++i) {
+    Layer* layer = LayerTreeHostCommon::FindLayerInSubtree(
+        root_layer_.get(), info->scrolls[i].layer_id);
     if (!layer)
       continue;
     if (layer == outer_viewport_scroll_layer_.get()) {
-      outer_viewport_scroll_delta += info.scrolls[i].scroll_delta;
+      outer_viewport_scroll_delta += info->scrolls[i].scroll_delta;
     } else if (layer == inner_viewport_scroll_layer_.get()) {
-      inner_viewport_scroll_delta += info.scrolls[i].scroll_delta;
+      inner_viewport_scroll_delta += info->scrolls[i].scroll_delta;
     } else {
       layer->SetScrollOffsetFromImplSide(layer->scroll_offset() +
-                                         info.scrolls[i].scroll_delta);
+                                         info->scrolls[i].scroll_delta);
     }
   }
 
   if (!inner_viewport_scroll_delta.IsZero() ||
-      !outer_viewport_scroll_delta.IsZero() || info.page_scale_delta != 1.f) {
+      !outer_viewport_scroll_delta.IsZero() || info->page_scale_delta != 1.f) {
     // SetScrollOffsetFromImplSide above could have destroyed the tree,
     // so re-get this layer before doing anything to it.
 
@@ -1058,11 +1071,11 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
           outer_viewport_scroll_layer_->scroll_offset() +
           outer_viewport_scroll_delta);
     }
-    ApplyPageScaleDeltaFromImplSide(info.page_scale_delta);
+    ApplyPageScaleDeltaFromImplSide(info->page_scale_delta);
 
     client_->ApplyScrollAndScale(
         inner_viewport_scroll_delta + outer_viewport_scroll_delta,
-        info.page_scale_delta);
+        info->page_scale_delta);
   }
 }