Add CSS Flexbox experiment to enums.xml

Design doc: https://goo.gle/devtools-flex

Bug: 1139945
Change-Id: I5506d17e453e2c36f632ebe580147882a1098d30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485268Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818890}

tools/metrics/histograms/enums.xml

@@ -16459,6 +16459,7 @@ metrics consent we also won't be able to send UMA metrics. -->
   <int value="33" label="cssGridFeatures"/>
   <int value="34" label="movableTabs"/>
   <int value="35" label="keyboardShortcutEditor"/>
+  <int value="36" label="cssFlexboxFeatures"/>
 </enum>
 
 <enum name="DevToolsGridOverlayOpenedFrom">