Avoid crashing the browser on truncated reads from the cache backend

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 42526547a35e185b6422722d7996e0a37f5e5db0..f040f6c3af5a3708e09e1dbfcd9662a14e703803 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -826,6 +826,14 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="HttpCache.TruncatedHeader" units="BooleanTruncated">
+  <summary>Whether cache headers were truncated before read.</summary>
+</histogram>
+
+<histogram name="HttpCache.TruncatedMetadata" units="BooleanTruncated">
+  <summary>Whether cache entry metadata was truncated before read.</summary>
+</histogram>
+
 <histogram name="Import.ImporterType.AutoImport" enum="ImporterType">
   <summary>The importer used on first run Auto Import.</summary>
 </histogram>
@@ -6381,6 +6389,11 @@ other types of suffix sets.
   <int value="1" label="Memory low, tabs discarded"/>
 </enum>
 
+<enum name="BooleanTruncated" type="int">
+  <int value="0" label="Was Not Truncated"/>
+  <int value="1" label="Was Truncated"/>
+</enum>
+
 <enum name="BooleanUsage" type="int">
   <int value="0" label="Not Used"/>
   <int value="1" label="Used"/>