Add detailed tracking for causes of invalid PageLoadTimings.

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 19f4b0c02e241c5abce4b81dbcd9f1e96b84acbf..edfc3ec11c156204db736334652cc6bfc7a8150a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -47035,6 +47035,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageLoad.Internal.PageLoadTimingStatus"
+    enum="PageLoadTimingStatus">
+  <owner>bmcquade@chromium.org</owner>
+  <summary>
+    The status of PageLoadTiming structs received from the render process over
+    IPC.
+  </summary>
+</histogram>
+
 <histogram name="PageLoad.Internal.Prerender" enum="BooleanHit">
   <owner>bmcquade@chromium.org</owner>
   <summary>
@@ -106478,6 +106487,30 @@ from previous Chrome versions.
   <int value="5" label="Successful first layout (backgrounded)"/>
 </enum>
 
+<enum name="PageLoadTimingStatus" type="int">
+  <summary>Status of PageLoadTimings received from the render process</summary>
+  <int value="0" label="Valid"/>
+  <int value="1" label="Empty timing"/>
+  <int value="2" label="Null navigation start"/>
+  <int value="3" label="Script load longer than parse duration"/>
+  <int value="4" label="Script execution longer than parse duration"/>
+  <int value="5"
+      label="Script load from doc.write longer than total script load"/>
+  <int value="6"
+      label="Script execution from doc.write longer than total script
+             execution"/>
+  <int value="7" label="Invalid order - response start / parse start"/>
+  <int value="8" label="Invalid order - parse start / parse stop"/>
+  <int value="9" label="Invalid order - parse stop / dom content loaded"/>
+  <int value="10" label="Invalid order - dom content loaded / load"/>
+  <int value="11" label="Invalid order - parse start / first layout"/>
+  <int value="12" label="Invalid order - first layout / first paint"/>
+  <int value="13" label="Invalid order - first paint / first text paint"/>
+  <int value="14" label="Invalid order - first paint / first image paint"/>
+  <int value="15" label="Invalid order - first paint / first contentful paint"/>
+  <int value="16" label="Invalid order - first paint / first meaningful paint"/>
+</enum>
+
 <enum name="PageScaleFactorRange" type="int">
   <int value="0" label="&lt;25%"/>
   <int value="1" label="25-49%"/>