[DevTools] Add user action for WebAuthn

Adds action for enabling virtual authenticator environment.

Used as metric to track use of WebAuthn DevTools front-end.

Related CL that implements metric: https://crrev.com/c/2333990

Design doc: https://docs.google.com/document/d/1A-cn0rxuHHAyGhywayk7xcTqSsR2X7va6H4kSMThOoY

Bug: 1034663
Change-Id: I2e4145e3bb1951968770971a3f0bace8a8403f7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332995Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Fawaz Mohammad <fawazm@google.com>
Cr-Commit-Position: refs/heads/master@{#794563}

tools/metrics/histograms/enums.xml

@@ -15982,6 +15982,7 @@ metrics consent we also won't be able to send UMA metrics. -->
   <int value="39" label="Tab moved to drawer"/>
   <int value="40" label="Tab moved to main panel"/>
   <int value="41" label="Capture CSS Overview clicked"/>
+  <int value="42" label="Virtual authenticator environment enabled"/>
 </enum>
 
 <enum name="DevToolsBackgroundService">