DevTools: Adding DevToolsAction enum state to record css var edits

Now that we have links in var() functions in the Styles panel of
DevTools to allow users to jump to where custom properties are defined
it becomes more interesting to track if people actually edit those
properties.

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/+/2458452/

Bug: 1124707
Change-Id: I07a798ee97a1266787aa0fc8a0bd07278348dd12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2458766
Commit-Queue: Patrick Brosset <patrick.brosset@microsoft.com>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Reviewed-by: Brandon Goddard <brgoddar@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#815551}

tools/metrics/histograms/enums.xml

@@ -16385,6 +16385,7 @@ metrics consent we also won't be able to send UMA metrics. -->
   <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"/>
+  <int value="48" label="Custom property value edited"/>
 </enum>
 
 <enum name="DevToolsBackgroundService">