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/+/1733037Reviewed-by: Lan Wei <lanwei@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684518}

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_ENABLED_BY_DEFAULT};
+                                             base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kCompositorTouchAction{"CompositorTouchAction",
                                            base::FEATURE_ENABLED_BY_DEFAULT};