Move logic to clear ExtensionAction values to ExtensionActionAPI

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 48805f790f52eecd3b9d68a541348f24dd84d72e..708b6a213fe60f1f5a04fff4f820db42ae3dcc01 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -7666,13 +7666,6 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
-<histogram name="Extensions.LoadPageAction">

[Mark P, 2014/08/21 20:55:18]

Was this histogram ever recorded?  If so, please mark it as obsolete.

[Devlin, 2014/08/21 21:12:29]

I believe it may have been recorded at one point, but it seems there is no
longer data available for it on the histogram site.  I originally chalked this
up to not having an owner - is there another reason the data wouldn't show up? 
(Do you know if the site's down?)

Reason being, if there really is no data, no reason to mark it as obsolete,
right?

[Mark P, 2014/08/21 21:19:38]

Right.  It appears to have never been recorded, ever.

-  <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
-  <summary>
-    The number of page action extensions loaded at browser startup.
-  </summary>
-</histogram>
-
 <histogram name="Extensions.LoadPlatformApp">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>The number of platform apps loaded at browser startup.</summary>