Implement a poor man's PostAfterStartupTask() function.

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 fc5557ea030a7d5d0acb711eb142c37eca705af5..3ee600220d2624c78ecd30fafe73de98fbbc086f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -247,6 +247,21 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="AfterStartupTasks.Count">

[Ilya Sherman, 2015/04/09 23:53:56]

Rather than creating a new "AfterStartupTasks" top-level group, could you please
declare this as a subgroup of a reasonable existing group?  The "Startup" group
seems like a pretty good choice, IMO.

+  <owner>michaeln@chromium.org</owner>
+  <summary>
+    The number of after startup tasks that were queued prior to startup

[Ilya Sherman, 2015/04/09 23:53:56]

nit: I think "after-startup" might be easier to read and parse than "after
startup"

+    completion.

[Ilya Sherman, 2015/04/09 23:53:56]

Please document when this is recorded.  I assume that it's at startup completion
time, but I'm not certain.

+  </summary>
+</histogram>
+
+<histogram name="AfterStartupTasks.DelayedUntilTime" units="milliseconds">
+  <owner>michaeln@chromium.org</owner>
+  <summary>
+    Time from the process creation until after startup tasks were run.

[Ilya Sherman, 2015/04/09 23:53:56]

Is this when the first task started to be run, or when the tasks completed, or
when all of them were started, but perhaps not yet completed?  Please clarify in
the description.

+  </summary>
+</histogram>
+
 <histogram name="Android.KernelVersion" enum="AndroidKernelVersion">
   <owner>rsesek@chromium.org</owner>
   <summary>