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..06c06e84ab0580d58bf3bd375b5afeb971f5c339 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);
@@ -116,6 +117,7 @@ class CONTENT_EXPORT DOMStorageArea
   FRIEND_TEST_ALL_PREFIXES(DOMStorageAreaTest, PurgeMemory);
   FRIEND_TEST_ALL_PREFIXES(DOMStorageAreaTest, RateLimiter);
   FRIEND_TEST_ALL_PREFIXES(DOMStorageContextImplTest, PersistentIds);
+  FRIEND_TEST_ALL_PREFIXES(DOMStorageContextImplTest, PurgeMemory);
   friend class base::RefCountedThreadSafe<DOMStorageArea>;
 
   // Used to rate limit commits.