Extend wheel scroll update latency histograms.

Specifically, this extends the following histograms:
- Event.Latency.ScrollUpdate.Wheel.HandledToRendererSwap2_Impl
- Event.Latency.ScrollUpdate.Wheel.HandledToRendererSwap2_Main
- Event.Latency.ScrollUpdate.Wheel.HandledToRendererSwap2

These are used to narrow down which part of scroll event dispatch was
slow when Event.Latency.ScrollUpdate.Wheel.TimeToScrollUpdateSwapBegin4
has a regression during input metrics triage. When we resolve the
discrepencies between these metrics and EventLatency metrics
(https://crbug.com/1079116) these can be replaced but these also serve
as a valuable comparison to find where the discrepencies are coming
from.

Bug: 1149530
Change-Id: I4826b65ab0363e5cd3ea1691580d091ebb0ea911
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626417
Commit-Queue: Robert Flack <flackr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845218}

tools/metrics/histograms/histograms_xml/event/histograms.xml

@@ -1339,11 +1339,14 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Event.Latency.ScrollUpdate.Wheel.HandledToRendererSwap2"
-    units="microseconds" expires_after="2020-12-27">
+    units="microseconds" expires_after="2021-06-16">
   <owner>flackr@chromium.org</owner>
+  <owner>input-dev@chromium.org</owner>
   <summary>
     Time between the ScrollUpdate gesture event with wheel source, is handled on
     main/impl thread (specified by suffix) and before renderer starts to swap.
+    This is recorded each time the user scrolls the mouse wheel when that frame
+    is swapped to the display.
 
     Team: input-dev@chromium.org.