Revert "Disable flag "UpdateHoverAtBeginFrame""

This reverts commit 95343512.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Disable flag "UpdateHoverAtBeginFrame"
> 
> Blink.MainFrame.StyleAndLayoutRatio metrics has a significant
> regression increase, we will disable flag "UpdateHoverAtBeginFrame" for
> now in M78 to help identify what is the reason to cause this
> regression.
> 
> TBR=bokan@chromium.org
> 
> Change-Id: Ic85d4c142a2a25928c3ec25a61de20649144ef50
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733037
> Reviewed-by: Lan Wei <lanwei@chromium.org>
> Commit-Queue: Lan Wei <lanwei@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#684518}

TBR=bokan@chromium.org,lanwei@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Id6ad8e351ed45dc60d21984ef49bcbbe45dd5b2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752765Reviewed-by: Lan Wei <lanwei@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686620}

ui/events/blink/blink_features.cc

@@ -16,7 +16,7 @@ const base::Feature kSendMouseLeaveEvents{"SendMouseLeaveEvents",
                                           base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kUpdateHoverAtBeginFrame{"UpdateHoverAtBeginFrame",
-                                             base::FEATURE_DISABLED_BY_DEFAULT};
+                                             base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kCompositorTouchAction{"CompositorTouchAction",
                                            base::FEATURE_ENABLED_BY_DEFAULT};