Add UMA histograms for the inter-process TimeTicks skew.

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 fb1a1d0bd2d615446f53471e964310647397cb38..87cf64f7ff6c2d3ed352401864df2872727899f4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -10182,6 +10182,30 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="InterProcessTimeTicks.BrowserAhead" units="milliseconds">
+  <owner>ppi@chromium.org</owner>
+  <summary>
+    Estimated additive skew between processes, recorded if the browser process

[pasko, 2014/08/18 09:30:34]

It is usually a good idea to mention _when_ the histogram is collected.

+    is ahead (higher TimeTicks value) than the renderer process.
+  </summary>
+</histogram>
+
+<histogram name="InterProcessTimeTicks.BrowserBehind" units="milliseconds">
+  <owner>ppi@chromium.org</owner>
+  <summary>
+    Estimated additive skew between processes, recorded if the browser process
+    is behind (lower TimeTicks value) than the renderer process.
+  </summary>
+</histogram>
+
+<histogram name="InterProcessTimeTicks.IsSkewAdditive">

[Alexei Svitkine (slow), 2014/08/15 16:54:34]

Nit: Specify an enum="Boolean" or a more specific variant.

[ppi, 2014/08/18 09:29:05]

Done.

+  <owner>ppi@chromium.org</owner>
+  <summary>
+    True iff the conversion from the browser process TimeTicks to renderer
+    process TimeTicks is done by adding a constant, without scaling.
+  </summary>
+</histogram>
+
 <histogram name="interstitial.authority_invalid_time" units="milliseconds">
   <obsolete>
     Removed on 8/1/13.
@@ -51452,6 +51476,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <affected-histogram name="PLT.PT_StartToFinish"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="InterProcessTimeTicksConversionType">
+  <owner>ppi@chromium.org</owner>
+  <suffix name="BrowserToRenderer"/>
+  <suffix name="RendererToBrowser"/>
+  <affected-histogram name="InterProcessTimeTicks.BrowserAhead"/>
+  <affected-histogram name="InterProcessTimeTicks.BrowserBehind"/>
+  <affected-histogram name="InterProcessTimeTicks.IsSkewAdditive"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="Interval" separator="_">
   <suffix name="Interval" label="Interval between two consecutive connects is"/>
   <affected-histogram name="Net.TCP_Connection_Latency"/>