Add UMA enum for Assistant palette tool

The internal name of the tool is "Metalayer"
(PaletteTrayOptions::PALETTE_METALAYER, see
ash/system/palette/palette_ids.h)

Bug: 750911
Change-Id: I56acd0a5e7571b9fab25d0d53053dca8680916d2
Reviewed-on: https://chromium-review.googlesource.com/595168Reviewed-by: Vadim Tryshev <vadimt@chromium.org>
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491039}

tools/metrics/histograms/enums.xml

@@ -28431,6 +28431,7 @@ from previous Chrome versions.
   <int value="5" label="Add new note"/>
   <int value="6" label="Magnifying glass mode"/>
   <int value="7" label="Laser pointer mode"/>
+  <int value="8" label="Assistant mode"/>
 </enum>
 
 <enum name="PanningModelType">