Add Full A11y Tree experiment to enums.xml

Bug: N/A
Change-Id: Id7ff65b9428c73e565561cd92f1fa375a0515048
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577076Reviewed-by: Meredith Lane <meredithl@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835131}

tools/metrics/histograms/enums.xml

@@ -16893,6 +16893,7 @@ metrics consent we also won't be able to send UMA metrics. -->
   <int value="39" label="APCA"/>
   <int value="40" label="cspViolationsView"/>
   <int value="41" label="fontEditor"/>
+  <int value="42" label="fullAccessibilityTree"/>
 </enum>
 
 <enum name="DevToolsGridOverlayOpenedFrom">