Purge browser cache for dom storage in a smarter way

content/browser/dom_storage/dom_storage_area.h

Index: content/browser/dom_storage/dom_storage_area.h
diff --git a/content/browser/dom_storage/dom_storage_area.h b/content/browser/dom_storage/dom_storage_area.h
index 9fc646be19bae8f38225e9204c780fb8ab094188..a8ffa7ccbef54f63d3022cd80704e6d604be30f6 100644
--- a/content/browser/dom_storage/dom_storage_area.h
+++ b/content/browser/dom_storage/dom_storage_area.h
@@ -18,6 +18,7 @@
 #include "base/strings/nullable_string16.h"
 #include "base/strings/string16.h"
 #include "content/common/content_export.h"
+#include "content/common/dom_storage/dom_storage_map.h"
 #include "content/common/dom_storage/dom_storage_types.h"
 #include "url/gurl.h"
 
@@ -30,7 +31,6 @@ class ProcessMemoryDump;
 namespace content {
 
 class DOMStorageDatabaseAdapter;
-class DOMStorageMap;
 class DOMStorageTaskRunner;
 class SessionStorageDatabase;
 
@@ -65,6 +65,7 @@ class CONTENT_EXPORT DOMStorageArea
 
   const GURL& origin() const { return origin_; }
   int64_t namespace_id() const { return namespace_id_; }
+  size_t map_usage_in_bytes() const { return map_ ? map_->bytes_used() : 0; }
 
   // Writes a copy of the current set of values in the area to the |map|.
   void ExtractValues(DOMStorageValuesMap* map);