Adds a new UMA histogram to measure the NTP load time since navigation

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 443b3af0ba1586ee176759131684aa17ba32c1aa..3a0ae783297afcd5f92a2c189a6f9a48d1fe4732 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -8879,9 +8879,8 @@ Therefore, the affected-histogram name has to have at least one dot in it.
 <histogram name="Extensions.FeatureProviderStaticInitTime" units="milliseconds">
   <owner>rkaplow@chromium.org</owner>
   <summary>
-    The amount of time that elapsed during
-    extensions::FeatureProvider::Static. Only measured while in the browser
-    process.
+    The amount of time that elapsed during extensions::FeatureProvider::Static.
+    Only measured while in the browser process.
   </summary>
 </histogram>
 
@@ -19591,8 +19590,8 @@ Therefore, the affected-histogram name has to have at least one dot in it.
     enum="QuicSessionLocations">
   <owner>rch@chromium.org</owner>
   <summary>
-    The location in quic_client_session.cc where a session is unexpectedly
-    not going away.
+    The location in quic_client_session.cc where a session is unexpectedly not
+    going away.
   </summary>
 </histogram>
 
@@ -23056,6 +23055,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="NewTabPage.LoadTime">
+  <owner>fserb@chromium.org</owner>
+  <summary>
+    Histogram of the time, in milliseconds, it took for the NTP to load all its
+    tiles since navigation start.
+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.MobilePromo" enum="NewTabPageMobilePromo">
   <obsolete>
     Deprecated on M33 with the change to native NTP.