Update MainThreadScrollingReason histogram enum

The newly added MainThreadScrollingReasons: kWheelEventHandlerRegion and
kTouchEventHandlerRegion cannot be displayed correctly on UMA. This CL
updates the histogram enum in order to fix this bug.

Bug: 861754
Change-Id: Ibd165ffe477cbc405d047bc820c77056d730ec6e
Reviewed-on: https://chromium-review.googlesource.com/c/1257549
Commit-Queue: Xianda Sun <sunxd@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596228}

tools/metrics/histograms/enums.xml

@@ -31245,6 +31245,8 @@ from previous Chrome versions.
   <int value="21" label="Has clip related property"/>
   <int value="22" label="Has box shadow from non-root layer"/>
   <int value="23" label="Is not stacking context"/>
+  <int value="24" label="Blocking wheel event handler region"/>
+  <int value="25" label="Blocking touch event handler region"/>
 </enum>
 
 <enum name="MakeChromeDefaultResult">