JankTracker: Use full precision when sweep line algorithm is enabled.

Also use std::unordered_map instead of std::map in BasicIntervals, which
is much faster.

Bug: 581518
Change-Id: If36a6a70bdf5a4d02d87c66a2ee8264441602388
Reviewed-on: https://chromium-review.googlesource.com/c/1394402Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619979}

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

@@ -43,7 +43,8 @@ class BasicIntervals {
 
  private:
   Vector<int> endpoints_;
-  std::map<int, unsigned> endpoint_to_index_;
+  // Avoid WTF::HashMap as key may be 0 or -1.
+  std::unordered_map<int, unsigned> endpoint_to_index_;
 
 #if DCHECK_IS_ON()
   bool has_index_ = false;

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

@@ -42,6 +42,9 @@ static float GetMoveDistance(const FloatRect& old_rect,
 }
 
 static float RegionGranularityScale(const IntRect& viewport) {
+  if (RuntimeEnabledFeatures::JankTrackingSweepLineEnabled())
+    return 1;
+
   return kRegionGranularitySteps /
          std::min(viewport.Height(), viewport.Width());
 }

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

@@ -51,6 +51,9 @@ TEST_F(JankTrackerTest, SimpleBlockMovement) {
 }
 
 TEST_F(JankTrackerTest, GranularitySnapping) {
+  if (RuntimeEnabledFeatures::JankTrackingSweepLineEnabled())
+    return;
+
   SetBodyInnerHTML(R"HTML(
     <style>
       #j { position: relative; width: 304px; height: 104px; }