Cross-origin iframes: collect data under hypothetical loading strategies

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 bc086e693b95ff6d0e20ac3857c051ddaa80dc9b..1fa86b3802767e31b83cd19047f73c676bc66cb0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -28503,6 +28503,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histogram name="Navigation.DeferredDocumentLoading.StatesV3"
     enum="DocumentStateForDeferredLoadingV3">
+  <obsolete>
+    Deprecated 11/2016 in favor of Navigation.DeferredDocumentLoading.StatesV4.
+  </obsolete>
   <owner>dgrogan@chromium.org</owner>
   <summary>
     Why and if cross-origin documents would be loaded if we were to defer
@@ -28512,6 +28515,19 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Navigation.DeferredDocumentLoading.StatesV4"
+    enum="DocumentStateForDeferredLoadingV4">
+  <owner>dgrogan@chromium.org</owner>
+  <summary>
+    Why and if cross-origin documents would be loaded if we were to defer
+    loading as long as possible. Changes from V3: replace detailed reasons with
+    summaries for four strategies. E.g. any document that would load under a
+    strategy of loading when a frame is two screens away is logged to the
+    WouldLoad2Screens AND WouldLoad3Screens bucket. We only record documents
+    below the current viewport, not to the right of it.
+  </summary>
+</histogram>
+
 <histogram name="Navigation.EngagementTime.HTTP" units="ms">
   <owner>felt@chromium.org</owner>
   <summary>
@@ -81982,6 +81998,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="7" label="WouldLoadVisible"/>
 </enum>
 
+<enum name="DocumentStateForDeferredLoadingV4" type="int">
+  <int value="0" label="Invalid"/>
+  <int value="1" label="Created"/>
+  <int value="2" label="WouldLoad3ScreensAway"/>
+  <int value="3" label="WouldLoad2ScreensAway"/>
+  <int value="4" label="WouldLoad1ScreensAway"/>
+  <int value="5" label="WouldVisible"/>
+  <int value="6" label="WouldLoadNoParent"/>
+</enum>
+
 <enum name="DocumentWriteGatedEvaluation" type="int">
   <int value="0" label="Script too long"/>
   <int value="1" label="No likely external script write"/>