Add histogram to Capture CSS Overview Panel Shown metrics

Bug: 1101779

Change-Id: I00214f6ff55b260b9cabad80ae9bc5ad93d0886b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2280967
Auto-Submit: Jecelyn Yeen‎ <jec@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785274}

tools/metrics/histograms/enums.xml

@@ -16027,6 +16027,7 @@ to ensure that the crash string is shown properly on the user-facing crash UI.
   <int value="36" label="Settings - Shortcuts"/>
   <int value="37" label="Drawer - Issues"/>
   <int value="38" label="Settings - Shortcuts (custom)"/>
+  <int value="39" label="CSS Overview"/>
 </enum>
 
 <enum name="DevToolsSetting">