Restore 3px movement threshold.

This partially reverts crrev.com/700726, which removed the threshold,
and caused significant perf benchmark regressions (see linked bugs).

The test that verified the existence of the threshold is not restored,
since we no longer intend to spec this behavior.

Bug: 1003893,1009569,1009641,1009801
Change-Id: I284e53ba1f4c4c36cfcb2b54eaf5122758530c95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848899Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704267}

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

@@ -28,7 +28,7 @@ namespace blink {
 
 static constexpr base::TimeDelta kTimerDelay =
     base::TimeDelta::FromMilliseconds(500);
-static const float kMovementThreshold = 1.0;  // CSS pixels.
+static const float kMovementThreshold = 3.0;  // CSS pixels.
 
 static FloatPoint LogicalStart(const FloatRect& rect,
                                const LayoutObject& object) {