back gesture: fix the uma metrics name mismatch.

Bug: 1071598
Change-Id: I950ebd930cc06de05c65426bfa14081dc1c58dad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153689Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759796}

ash/wm/gestures/back_gesture/back_gesture_metrics.cc

@@ -17,13 +17,13 @@
 namespace ash {
 
 constexpr char kBackGestureStartScenarioHistogram[] =
-    "Tablet.BackGesture.StartScenario";
+    "Ash.BackGesture.StartScenarioType";
 
 constexpr char kBackGestureEndScenarioHistogram[] =
-    "Tablet.BackGesture.EndScenario";
+    "Ash.BackGesture.EndScenarioType";
 
 constexpr char kBackGestureUnderneathWindowTypeHistogram[] =
-    "Tablet.BackGesture.UnderneathWindowType";
+    "Ash.BackGesture.UnderneathWindowType";
 
 BackGestureStartScenarioType GetStartScenarioType(
     bool dragged_from_splitview_divider,