[Offline Pages] Adds UMA for result of attempts to immediately start background loading.

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 943d6a4f2ba263128b9e46be39c1290cc8bc88bc..c100a2a863391ab25d2102106ea6611c0ad5a227 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -38406,6 +38406,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="OfflinePages.Background.ImmediateStartStatus"
+    enum="OfflinePagesBackgroundImmediateStartStatus">
+  <owner>dougarnett@chromium.org</owner>
+  <summary>
+    Status of attempts to immediately start offlining a page in the background

[Mark P, 2016/10/06 23:11:36]

nit: attempts -> attempt
(each status is for a single attempt)

[dougarnett, 2016/10/07 00:18:56]

Done.

+    while application is still concurrently using the foreground. This may be
+    attempted when an web page download is initially requested or resumed.
+  </summary>
+</histogram>
+
 <histogram name="OfflinePages.Background.OfflinerRequestStatus"
     enum="OfflinePagesBackgroundOfflinerRequestStatus">
   <owner>dougarnett@chromium.org</owner>
@@ -91557,6 +91567,16 @@ value.
   <int value="11" label="Unknown net error, showing error page"/>
 </enum>
 
+<enum name="OfflinePagesBackgroundImmediateStartStatus" type="int">
+  <int value="0" label="Started"/>
+  <int value="1" label="Busy"/>
+  <int value="2" label="Not accepted by loader"/>
+  <int value="3" label="No connection"/>
+  <int value="4"
+      label="Weak connection (according to network quality estimator)"/>
+  <int value="5" label="Not started on svelte device"/>
+</enum>
+
 <enum name="OfflinePagesBackgroundOfflinerRequestStatus" type="int">
 <!-- Generated from components/offline_pages/background/offliner.h -->