Add histograms for Devtools Experiments

This CL introduces 3 histograms around devtools experiments:
ExperimentEnabled - fired when users turn on an experiment
ExperimentDisabled - fired when users disable an experiment
ExperimentEnabledAtLaunch - fires on Devtools launch for each
experiment that is enabled.

These will allow us to gain better understanding of how users
interact with devtools experiments.

Explainer:
https://docs.google.com/document/d/1pcvN11C4onSXqOE0dTjUUHodMj55bRVl_5_e3kMg_rs/edit?ts=5f2329dc#

Corresponding backend CL: https://crrev.com/c/2341215

Bug: 1107636
Change-Id: I4ae72881b6409d25204fea0335f6796161cc61c0
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2341950
Commit-Queue: Brandon Goddard <[email protected]>
Reviewed-by: Shane Clifford <[email protected]>
Reviewed-by: Kalon Hinds <[email protected]>
Reviewed-by: Patrick Brosset <[email protected]>
Reviewed-by: Peter Marshall <[email protected]>
Reviewed-by: Jack Lynch <[email protected]>
diff --git a/front_end/devtools_compatibility.js b/front_end/devtools_compatibility.js
index 815a5da..5a3205a 100644
--- a/front_end/devtools_compatibility.js
+++ b/front_end/devtools_compatibility.js
@@ -370,6 +370,9 @@
     KeybindSetSettingChanged: 'DevTools.KeybindSetSettingChanged',
     DualScreenDeviceEmulated: 'DevTools.DualScreenDeviceEmulated',
     CSSGridSettings: 'DevTools.CSSGridSettings',
+    ExperimentEnabledAtLaunch: 'DevTools.ExperimentEnabledAtLaunch',
+    ExperimentEnabled: 'DevTools.ExperimentEnabled',
+    ExperimentDisabled: 'DevTools.ExperimentDisabled',
   };
 
   /**