DevTools: Adding DevToolsAction enum state to record clicks in var()

Links now appear in var() functions in the Styles panel of DevTools to
allow users to jump to where custom properties are defined.

This change adds a new state to the DevToolsAction enum so we can record
when users click these links.

Frontend CL: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2451605/

Bug: 1124707
Change-Id: Ib38e57689973a94cf68837234d1bf66653e8d5fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451925Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Patrick Brosset <patrick.brosset@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#814809}

tools/metrics/histograms/enums.xml

@@ -16381,6 +16381,7 @@ metrics consent we also won't be able to send UMA metrics. -->
   <int value="44" label="User keyboard shortcut added"/>
   <int value="45" label="Keyboard shortcut removed"/>
   <int value="46" label="Keyboard shortcut modified"/>
+  <int value="47" label="Link to custom property in var() clicked"/>
 </enum>
 
 <enum name="DevToolsBackgroundService">