DevTools: add source order histogram

This CL adds backend support for new devtools user action
for activating the source order viewer.

Frontend CL: https://crrev.com/c/2353377

Bug: 1094406
Change-Id: I49ee7e3d724ff67f131c9e1098d098830847722b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351391Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Brandon Goddard <brgoddar@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#797813}

tools/metrics/histograms/enums.xml

@@ -16002,6 +16002,7 @@ metrics consent we also won't be able to send UMA metrics. -->
   <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"/>
+  <int value="43" label="Source Order Overlay Activated"/>
 </enum>
 
 <enum name="DevToolsBackgroundService">