Add UMA histogram for suppressed dialogs on prerender swap.

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 56cefde47c04a6f1252816e5f745b87e3031a35d..af7e7940f0a5f41cda2a992d402281b7494dc14b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13734,7 +13734,16 @@ other types of suffix sets.
   </summary>
 </histogram>
 
-<histogram name="Prerender.TabContentsDeleterTimeout">
+<histogram name="Prerender.TabContentsDeleterSuppressedDialog"
+    enum="BooleanSuppressed">
+  <summary>
+    A boolean that indicates how often we suppress a dialog from a tab when
+    swapping it with a prerender.
+  </summary>
+</histogram>
+
+<histogram name="Prerender.TabContentsDeleterTimeout"
+    enum="BooleanCloseTimeout">
   <summary>
     A boolean that indicates how often we fail to delete an old prerendered tab
     before the timeout.
@@ -20215,6 +20224,11 @@ other types of suffix sets.
   <int value="1" label="Attempted"/>
 </enum>
 
+<enum name="BooleanCloseTimeout" type="int">
+  <int value="0" label="Closed normally"/>
+  <int value="1" label="Timed out"/>
+</enum>
+
 <enum name="BooleanCommonNameMatch" type="int">
   <int value="0" label="subjectAltName used"/>
   <int value="1" label="Common Name used"/>
@@ -20310,6 +20324,11 @@ other types of suffix sets.
   <int value="1" label="Success"/>
 </enum>
 
+<enum name="BooleanSuppressed" type="int">
+  <int value="0" label="No suppressions"/>
+  <int value="1" label="Suppressed"/>
+</enum>
+
 <enum name="BooleanTabDiscard" type="int">
   <int value="0" label="Memory OK, no discards"/>
   <int value="1" label="Memory low, tabs discarded"/>