[Sessions] Add detailed logging of SessionRestore events.

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 b13b369b4f61b5c71b0575e4c7a6cbc9620bfcd9..276b6c8a5eaf6327cff81806cc598f1d0baeebd5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -36036,6 +36036,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>How did the user interact with the SessionCrashed Bubble?</summary>
 </histogram>
 
+<histogram name="SessionRestore.Actions" enum="SessionRestoreActions">
+  <owner>chrisha@chromium.org</owner>
+  <summary>
+    The actions that have occurred in a session restore timeline. These are to
+    be interpreted as raw event counts. Tabs are almost certainly deferred due
+    to the existence memory pressure, but this may not always be the case.
+  </summary>
+</histogram>
+
 <histogram name="SessionRestore.AllTabsLoaded" units="milliseconds">
   <owner>jeremy@chromium.org</owner>
   <owner>sky@chromium.org</owner>
@@ -36166,6 +36175,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </details>
 </histogram>
 
+<histogram name="SessionRestore.TabActions" enum="SessionRestoreTabActions">
+  <owner>chrisha@chromium.org</owner>
+  <summary>
+    A breakdown of exactly how different tabs were treated during session
+    restore events.
+  </summary>
+</histogram>
+
 <histogram name="SessionRestore.TabClosedLongPeriod">
   <owner>jeremy@chromium.org</owner>
   <owner>sky@chromium.org</owner>
@@ -36183,6 +36200,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="SessionRestore.TabCount" units="tabs">
+  <owner>chrisha@chromium.org</owner>
+  <summary>
+    The number of tabs involved in a single session restore event.
+  </summary>
+</histogram>
+
 <histogram name="SessionStorageDatabase.Open" enum="SessionStorageDatabaseOpen">
   <owner>michaeln@chromium.org</owner>
   <summary>
@@ -62689,6 +62713,18 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="7" label="The bar with UMA opt-in option was shown."/>
 </enum>
 
+<enum name="SessionRestoreActions" type="int">
+  <int value="0" label="A session restore was started"/>
+  <int value="1" label="A session restore deferred one or more tabs"/>
+</enum>
+
+<enum name="SessionRestoreTabActions" type="int">
+  <int value="0" label="A tab was created"/>
+  <int value="1" label="A tab's content was automatically loaded"/>
+  <int value="2" label="The loading of a tab's content was deferred"/>
+  <int value="3" label="A deferred tab's content was loaded via user action"/>
+</enum>
+
 <enum name="SessionStartupPref" type="int">
   <int value="0" label="Open home page (unused)"/>
   <int value="1" label="Continue from last opened pages"/>