SimpleCache: read small files all at once.

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 77d76510de280610c33549d878cc48a962f1d8b6..88cc8ad00c1e2fcf09f2ee1c9b22f78a15d63736 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -75197,6 +75197,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>The outcome of Entry::ReadData in the simple cache.</summary>
 </histogram>
 
+<histogram base="true" name="SimpleCache.ReadStream1FromPrefetched"
+    enum="Boolean">
+  <owner>morlovich@chromium.org</owner>
+  <summary>
+    Whether a read from stream 1 (conventionally used for payload body) was
+    satisfied from prefetched data. Reported only on the first read operation on
+    the stream (including if there are multiple readers, or even some writers).
+  </summary>
+</histogram>
+
 <histogram base="true" name="SimpleCache.StaleIndexExtraEntryCount"
     units="entries">
   <owner>gavinp@chromium.org</owner>
@@ -75279,6 +75289,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram base="true" name="SimpleCache.SyncOpenDidPrefetch" enum="Boolean">
+  <owner>morlovich@chromium.org</owner>
+  <summary>
+    Whether an attempt was made to prefetch the entire file when executing
+    disk_cache::Backend::OpenEntry.
+  </summary>
+</histogram>
+
 <histogram base="true" name="SimpleCache.SyncOpenEntryAge" units="hours">
   <owner>gavinp@chromium.org</owner>
   <summary>
@@ -92595,6 +92613,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <affected-histogram name="SimpleCache.QueueLatency.OpenEntry"/>
   <affected-histogram name="SimpleCache.ReadIsParallelizable"/>
   <affected-histogram name="SimpleCache.ReadResult"/>
+  <affected-histogram name="SimpleCache.ReadStream1FromPrefetched"/>
   <affected-histogram name="SimpleCache.StaleIndexExtraEntryCount"/>
   <affected-histogram name="SimpleCache.StaleIndexMissedEntryCount"/>
   <affected-histogram name="SimpleCache.StaleIndexQuality"/>
@@ -92609,6 +92628,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <affected-histogram name="SimpleCache.SyncCreateResult_WithIndex"/>
   <affected-histogram name="SimpleCache.SyncCreateResult_WithoutIndex"/>
   <affected-histogram name="SimpleCache.SyncKeySHA256Result"/>
+  <affected-histogram name="SimpleCache.SyncOpenDidPrefetch"/>
   <affected-histogram name="SimpleCache.SyncOpenEntryAge"/>
   <affected-histogram name="SimpleCache.SyncOpenPlatformFileError"/>
   <affected-histogram name="SimpleCache.SyncOpenPlatformFileError_WithIndex"/>