Add UMA histograms for the inter-process TimeTicks skew.

content/child/resource_dispatcher.cc

Index: content/child/resource_dispatcher.cc
diff --git a/content/child/resource_dispatcher.cc b/content/child/resource_dispatcher.cc
index 3ca9668c246a0a3fca0bc7ab1d6bee6275f9ed82..18efc6caff73744d1837b1548ca6c541935adffe 100644
--- a/content/child/resource_dispatcher.cc
+++ b/content/child/resource_dispatcher.cc
@@ -785,6 +785,23 @@ void ResourceDispatcher::ToResourceResponseInfo(
   RemoteToLocalTimeTicks(converter, &load_timing->send_start);
   RemoteToLocalTimeTicks(converter, &load_timing->send_end);
   RemoteToLocalTimeTicks(converter, &load_timing->receive_headers_end);
+
+  // Collect UMA on the inter-process skew.
+  if (converter.IsSkewAdditiveForMetrics()) {
+    UMA_HISTOGRAM_BOOLEAN(
+        "InterProcessTimeTicks.IsSkewAdditive_BrowserToRenderer", true);

[Alexei Svitkine (slow), 2014/08/15 16:54:33]

Nit: Extract the if condition outside the if into a local var and log this
histogram only once outside the loop. (Will reduce number of macro invocations,
since each one expands to a bunch of code and adds to the binary bloat.)

Same for the other code.

[ppi, 2014/08/18 09:29:04]

Done.

+    base::TimeDelta skew = converter.GetSkewForMetrics();
+    if (skew >= base::TimeDelta()) {
+      UMA_HISTOGRAM_TIMES(
+          "InterProcessTimeTicks.BrowserAhead_BrowserToRenderer", skew);
+    } else {
+      UMA_HISTOGRAM_TIMES(
+          "InterProcessTimeTicks.BrowserBehind_BrowserToRenderer", -skew);
+    }
+  } else {
+    UMA_HISTOGRAM_BOOLEAN(
+        "InterProcessTimeTicks.IsSkewAdditive_BrowserToRenderer", false);
+  }
 }
 
 base::TimeTicks ResourceDispatcher::ToRendererCompletionTime(