Report layout jank fractions to the WebLocalFrameClient.

This will allow the page_load_metrics component to observe the jank.

Also address nits in JankTracker::NotifyPrePaintFinished (empty viewport
check, DCHECK for non-zero jank fraction).

Bug: 581518
Change-Id: I22eab226dac31bd1b0ce284240bbca019ce49868
Reviewed-on: https://chromium-review.googlesource.com/c/1280616Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599912}

third_party/blink/public/web/web_local_frame_client.h

@@ -666,6 +666,9 @@ class BLINK_EXPORT WebLocalFrameClient {
   virtual void DidObserveNewCssPropertyUsage(int /*css_property*/,
                                              bool /*is_animated*/) {}
 
+  // Reports that visible elements in the frame shifted (bit.ly/lsm-explainer).
+  virtual void DidObserveLayoutJank(double jank_fraction) {}
+
   // Script notifications ------------------------------------------------
 
   // Notifies that a new script context has been created for this frame.

third_party/blink/renderer/core/exported/local_frame_client_impl.cc

@@ -735,6 +735,11 @@ void LocalFrameClientImpl::DidObserveNewCssPropertyUsage(int css_property,
   }
 }
 
+void LocalFrameClientImpl::DidObserveLayoutJank(double jank_fraction) {
+  if (WebLocalFrameClient* client = web_frame_->Client())
+    client->DidObserveLayoutJank(jank_fraction);
+}
+
 bool LocalFrameClientImpl::ShouldTrackUseCounter(const KURL& url) {
   if (web_frame_->Client())
     return web_frame_->Client()->ShouldTrackUseCounter(url);

third_party/blink/renderer/core/exported/local_frame_client_impl.h

@@ -148,6 +148,7 @@ class LocalFrameClientImpl final : public LocalFrameClient {
   void DidObserveLoadingBehavior(WebLoadingBehaviorFlag) override;
   void DidObserveNewFeatureUsage(mojom::WebFeature) override;
   void DidObserveNewCssPropertyUsage(int, bool) override;
+  void DidObserveLayoutJank(double jank_fraction) override;
   bool ShouldTrackUseCounter(const KURL&) override;
   void SelectorMatchChanged(const Vector<String>& added_selectors,
                             const Vector<String>& removed_selectors) override;

third_party/blink/renderer/core/frame/local_frame_client.h

@@ -223,6 +223,10 @@ class CORE_EXPORT LocalFrameClient : public FrameClient {
   // process for histograms.
   virtual void DidObserveNewCssPropertyUsage(int /*css_property*/,
                                              bool /*is_animated*/) {}
+
+  // Reports that visible elements in the frame shifted (bit.ly/lsm-explainer).
+  virtual void DidObserveLayoutJank(double jank_fraction) {}
+
   // Will be called by a Page upon DidCommitLoad, deciding whether to track
   // UseCounter usage or not based on its url.
   virtual bool ShouldTrackUseCounter(const KURL&) { return true; }

third_party/blink/renderer/core/layout/jank_tracker.cc

@@ -6,6 +6,7 @@
 
 #include "third_party/blink/renderer/core/frame/local_dom_window.h"
 #include "third_party/blink/renderer/core/frame/local_frame.h"
+#include "third_party/blink/renderer/core/frame/local_frame_client.h"
 #include "third_party/blink/renderer/core/frame/location.h"
 #include "third_party/blink/renderer/core/layout/layout_object.h"
 #include "third_party/blink/renderer/core/layout/layout_view.h"
@@ -171,9 +172,14 @@ void JankTracker::NotifyPrePaintFinished() {
   IntRect viewport = frame_view_->GetScrollableArea()->VisibleContentRect();
   double granularity_scale = RegionGranularityScale(viewport);
   viewport.Scale(granularity_scale);
-  double viewport_area = double(viewport.Width()) * double(viewport.Height());
 
+  if (viewport.IsEmpty())
+    return;
+
+  double viewport_area = double(viewport.Width()) * double(viewport.Height());
   double jank_fraction = region_.Area() / viewport_area;
+  DCHECK_GT(jank_fraction, 0);
+
   score_ += jank_fraction;
 
   DVLOG(1) << "viewport " << (jank_fraction * 100)
@@ -184,7 +190,9 @@ void JankTracker::NotifyPrePaintFinished() {
                        PerFrameTraceData(jank_fraction, granularity_scale),
                        "frame", ToTraceValue(&frame_view_->GetFrame()));
 
-  if (RuntimeEnabledFeatures::LayoutJankAPIEnabled() && jank_fraction > 0 &&
+  frame_view_->GetFrame().Client()->DidObserveLayoutJank(jank_fraction);
+
+  if (RuntimeEnabledFeatures::LayoutJankAPIEnabled() &&
       frame_view_->GetFrame().DomWindow()) {
     WindowPerformance* performance =
         DOMWindowPerformance::performance(*frame_view_->GetFrame().DomWindow());