[OfflinePages] Adds TimeToStart UMA for background load requests.

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 b8f5b469c231f8e8ff0679bd3969ffb42985be8c..0ce020013beeedb093bdf4ccb5a35163e576bcc6 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -38982,6 +38982,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="OfflinePages.Background.TimeToStart" units="ms">
+  <owner>dougarnett@chromium.org</owner>
+  <summary>
+    Time from background request until processing it was first started. This is
+    for non-svelte devices.
+  </summary>
+</histogram>
+
+<histogram name="OfflinePages.Background.TimeToStart.Svelte" units="ms">
+  <owner>dougarnett@chromium.org</owner>
+  <summary>
+    Time from background request until processing it was first started on svelte
+    device. Svelte is determined by base::SysInfo::IsLowEnd().
+  </summary>
+</histogram>
+
 <histogram name="OfflinePages.Background.UnsupportedScheme.ConnectionType"
     enum="NetworkConnectionType">
   <owner>dougarnett@chromium.org</owner>
@@ -108280,6 +108296,8 @@ value.
   <affected-histogram name="OfflinePages.Background.OfflinerRequestStatus"/>
   <affected-histogram name="OfflinePages.Background.TimeToCanceled"/>
   <affected-histogram name="OfflinePages.Background.TimeToSaved"/>
+  <affected-histogram name="OfflinePages.Background.TimeToStart"/>
+  <affected-histogram name="OfflinePages.Background.TimeToStart.Svelte"/>
   <affected-histogram name="OfflinePages.DeletePage.AccessCount"/>
   <affected-histogram name="OfflinePages.DeletePage.LastOpenToCreated"/>
   <affected-histogram name="OfflinePages.DeletePage.PageSize"/>