Add StaleWhileRevalidate paint timing histograms.

Generally PaintTiming histograms haven't been added to histograms.xml
it appears. So this wasn't done in http://crrev.com/e5d647d0

But chirp is complaining about the missing histograms so corrrect that.

BUG=348877

Change-Id: I2605949ae729b93852147716fed6d4e15064688f
Reviewed-on: https://chromium-review.googlesource.com/c/1326802
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607046}

tools/metrics/histograms/histograms.xml

@@ -73202,6 +73202,29 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram base="true"
+    name="PageLoad.Clients.StaleWhileRevalidate.PaintTiming.FirstContentfulPaint"
+    units="ms" expires_after="M75">
+  <owner>dtapuska@chromium.org</owner>
+  <owner>kenjibaheux@google.com</owner>
+  <summary>
+    Measures the time from navigation start to first contentful paint. Recorded
+    for pages that contain a resource that may benefit from stale while
+    revalidate.
+  </summary>
+</histogram>
+
+<histogram base="true"
+    name="PageLoad.Clients.StaleWhileRevalidate.PaintTiming.ParseToFirstContentfulPaint"
+    units="ms" expires_after="M75">
+  <owner>dtapuska@chromium.org</owner>
+  <owner>kenjibaheux@google.com</owner>
+  <summary>
+    Measures the time from parse start to first contentful paint. Recorded for
+    pages that contain a resource that may benefit from stale while revalidate.
+  </summary>
+</histogram>
+
 <histogram name="PageLoad.Clients.SubresourceFilter.ActivationDecision"
     enum="SubresourceFilterActivationDecision">
   <obsolete>
@@ -136589,6 +136612,16 @@ uploading your change for review.
   <affected-histogram name="PLT.BeginToFinish_NormalLoad"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="StaleWhileRevalidate" separator=".">
+  <suffix name="Cache"/>
+  <suffix name="Network"/>
+  <suffix name="StaleCache"/>
+  <affected-histogram
+      name="PageLoad.Clients.StaleWhileRevalidate.PaintTiming.FirstContentfulPaint"/>
+  <affected-histogram
+      name="PageLoad.Clients.StaleWhileRevalidate.PaintTiming.ParseToFirstContentfulPaint"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="StartupProfilingAbandonState" separator="_">
   <suffix name="NoLoad" label="Abandoned before first main frame load."/>
   <suffix name="NoPaint" label="Abandoned before first paint."/>