Add uma stats to help evaluate the impact of changes to the quota allocation logic.

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 fa74d34a3660ccd68011df6933550b8012829403..7ac123c6ad812132ae7c31c6e0291657493868f4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -71892,6 +71892,20 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram base="true" name="Storage.BytesRead" units="bytes">
+  <owner>michaeln@chromium.org</owner>
+  <summary>
+    The number of bytes read. Recorded on each read.
+  </summary>
+</histogram>
+
+<histogram base="true" name="Storage.BytesWritten" units="bytes">
+  <owner>michaeln@chromium.org</owner>
+  <summary>
+    The number of bytes written. Recorded on each write.
+  </summary>
+</histogram>
+
 <histogram name="Storage.ImportantSites.CBDChosenReason"
     enum="ImportantSitesReason">
   <owner>dmurph@chromium.org</owner>
@@ -92591,6 +92605,20 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <affected-histogram name="Storage.Blob.SizeEvictedToDiskInKB"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="Storage.Bytes" separator=".">
+  <suffix name="DiskCache.AppCache" label="AppCache usage."/>
+  <suffix name="DiskCache.CacheStorage" label="CacheStorage usage."/>
+  <suffix name="DiskCache.ServiceWorker"
+      label="ServiceWorker scriptcache usage."/>
+  <suffix name="LevelDBEnv" label="Undifferentiated leveldb usage."/>
+  <suffix name="LevelDBEnv.IDB" label="IndexedDB usage."/>
+  <suffix name="LevelDBEnv.ServiceWorker"
+      label="ServiceWorker database usage."/>
+  <suffix name="MojoLevelDBEnv" label="Mojo leveldb component usage."/>
+  <affected-histogram name="Storage.BytesRead"/>
+  <affected-histogram name="Storage.BytesWritten"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="StunPingInternal" separator=".">
   <suffix name="0ms" label="0ms between requests"/>
   <suffix name="5ms" label="5ms between requests"/>