Initial skeleton of custom layouts for Web Notifications.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 132ec4ad6ecfc4e6222a0494a013c5fcbb2f5573..74c0b67d1293f9f67237d53e6af2530399223c1d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -63906,6 +63906,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="-979057409" label="enable-seccomp-filter-sandbox"/>
   <int value="-979034258" label="disable-ntp-other-sessions-menu"/>
   <int value="-978700508" label="disable-simplified-fullscreen-ui"/>
+  <int value="-976585018" label="disable-notifications-custom-layouts"/>
   <int value="-972425050" label="gesture-editing"/>
   <int value="-964676765" label="enable-accelerated-mjpeg-decode"/>
   <int value="-951394314" label="top-chrome-md"/>
@@ -63967,6 +63968,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="-462205750" label="enable-service-worker-sync"/>
   <int value="-430360431" label="disable-password-generation"/>
   <int value="-418868128" label="enable-experimental-web-platform-features"/>
+  <int value="-407298147" label="enable-notifications-custom-layouts"/>
   <int value="-385337473" label="enable-fast-unload"/>
   <int value="-384589459" label="disable-supervised-user-safesites"/>
   <int value="-378033324" label="disable-win32k-renderer-lockdown"/>