[Azure Pipelines] Enable experimental Server Timing in Safari TP

Like web-platform-tests/results-collection#621.

tools/ci/azure/install_safari.yml

@@ -1,19 +1,19 @@
 parameters:
   channel: preview
 
+# Should match https://web-platform-tests.org/running-tests/safari.html
 steps:
 - ${{ if eq(parameters.channel, 'preview') }}:
   - script: |
       # This is equivalent to `Homebrew/homebrew-cask-versions/safari-technology-preview`,
       # but the raw URL is used to bypass caching.
       HOMEBREW_NO_AUTO_UPDATE=1 brew cask install https://raw.githubusercontent.com/Homebrew/homebrew-cask-versions/master/Casks/safari-technology-preview.rb
-      # https://web-platform-tests.org/running-tests/safari.html
       sudo "/Applications/Safari Technology Preview.app/Contents/MacOS/safaridriver" --enable
       defaults write com.apple.Safari WebKitJavaScriptCanOpenWindowsAutomatically 1
+      defaults write com.apple.Safari ExperimentalServerTimingEnabled -bool true
     displayName: 'Install Safari Technology Preview'
 - ${{ if eq(parameters.channel, 'stable') }}:
   - script: |
-      # https://web-platform-tests.org/running-tests/safari.html
       sudo safaridriver --enable
       defaults write com.apple.Safari WebKitJavaScriptCanOpenWindowsAutomatically 1
     displayName: 'Configure Safari'