Log pref file size histogram on read rather than on write.

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 2e3627dec342cb5337ccf17b8f1aa0da6dd82600..2bcd9938c4546ff7faa256c080152701b4e8a437 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -32543,7 +32543,18 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Settings.JsonDataReadSizeKilobytes" units="KB">
+  <owner>gab@chromium.org</owner>
+  <summary>
+    The size (in kilobytes) of the JSON settings read from disk on startup.
+    Suffixed with the name of the corresponding JSON file.
+  </summary>
+</histogram>
+
 <histogram name="Settings.JsonDataSizeKilobytes" units="KB">
+  <obsolete>
+    Deprecated 02/2015. Replaced by Settings.JsonDataReadSizeKilobytes.
+  </obsolete>

[gab, 2015/01/30 13:25:27]

@asvitkine: it seemed more appropriate to deprecate the old histogram and create
a new one (if anything this may even help you clean up the logs cluster?), or do
you think we should just re-use the old one given it's really the same value
just reported differently?

   <owner>gab@chromium.org</owner>
   <summary>
     The size of the JSON settings content about to be written to disk in
@@ -61877,7 +61888,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <suffix name="Local_State" label="Local State file"/>
   <suffix name="Preferences" label="Preferences file"/>
   <suffix name="Secure_Preferences" label="Secure Preferences file"/>
-  <affected-histogram name="Settings.JsonDataSizeKilobytes"/>

[Alexei Svitkine (slow), 2015/01/30 13:49:03]

Keep this here, but mark the other histogram as <obsolete>.

[gab, 2015/01/30 17:33:37]

Done.

+  <affected-histogram name="Settings.JsonDataReadSizeKilobytes"/>
 </histogram_suffixes>
 
 <histogram_suffixes name="Prefetch">