Purge browser cache for dom storage in a smarter way

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 faf474fbf88f64e2dc387e6fb05c4e7d0a6ab72c..3712c43bfc39c99dba4cf95a297cfbd49c79a900 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -19558,6 +19558,20 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Records events related to local discovery notifications.</summary>
 </histogram>
 
+<histogram name="LocalStorage.BrowserLocalStorageCachePurgedInKB" units="KB">
+  <owner>ssid@chromium.org</owner>
+  <summary>
+    Total size of the HTML5 LocalStorage browser-side cache purged in KB.

[Mark P, 2016/05/23 18:27:15]

Please briefly mention when this tends to happen. 

[ssid, 2016/05/23 18:42:17]

Sorry I was thinking it should be a single line description. Fixed them, wdyt?

+  </summary>
+</histogram>
+
+<histogram name="LocalStorage.BrowserLocalStorageCacheSizeInKB" units="KB">
+  <owner>ssid@chromium.org</owner>
+  <summary>
+    Total size of the HTML5 LocalStorage DB(s) in KB in the browser-side cache.

[Mark P, 2016/05/23 18:27:14]

Please explicitly describe when this is emitted.

Also, please change "DB(s)" to "DBs" or "databases".  Writing it like "DB(s)"
make me think this function is emitted once for each database, which isn't
right.

[ssid, 2016/05/23 18:42:17]

Done.

+  </summary>
+</histogram>
+
 <histogram name="LocalStorage.BrowserLocalStorageSizeInKB" units="KB">
   <owner>michaeln@chromium.org</owner>
   <summary>