Add TRACE logging for browser controls

Needed to debug unexplained behavior seen in browser controls that
don't reproduce in a local Chrome build.

Bug: 861618
Change-Id: I9a6769cd47a3329313628bd714039ec532fc3d94
Reviewed-on: https://chromium-review.googlesource.com/c/1351272Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610963}

cc/input/browser_controls_offset_manager.cc

@@ -94,6 +94,10 @@ void BrowserControlsOffsetManager::UpdateBrowserControlsState(
   DCHECK(!(constraints == BrowserControlsState::kHidden &&
            current == BrowserControlsState::kShown));
 
+  TRACE_EVENT2("cc", "BrowserControlsOffsetManager::UpdateBrowserControlsState",
+               "constraints", static_cast<int>(constraints), "current",
+               static_cast<int>(current));
+
   // If the constraints have changed we need to inform Blink about it since
   // that'll affect main thread scrolling as well as layout.
   if (permitted_state_ != constraints) {
@@ -114,6 +118,7 @@ void BrowserControlsOffsetManager::UpdateBrowserControlsState(
       current == BrowserControlsState::kHidden)
     final_shown_ratio = 0.f;
   if (final_shown_ratio == TopControlsShownRatio()) {
+    TRACE_EVENT_INSTANT0("cc", "Ratio Unchanged", TRACE_EVENT_SCOPE_THREAD);
     ResetAnimations();
     return;
   }

cc/trees/layer_tree_impl.cc

@@ -969,6 +969,8 @@ bool LayerTreeImpl::ClampBrowserControlsShownRatio() {
 }
 
 bool LayerTreeImpl::SetCurrentBrowserControlsShownRatio(float ratio) {
+  TRACE_EVENT1("cc", "LayerTreeImpl::SetCurrentBrowserControlsShownRatio",
+               "ratio", ratio);
   bool changed = top_controls_shown_ratio_->SetCurrent(ratio);
   changed |= ClampBrowserControlsShownRatio();
   return changed;