buffet: Extract StorageInterface and implementations into separate files

This will make it easy to reuse file storage and memory storage
implementations in the forthcoming state aggregator.

BUG=chromium:369322
TEST=Unittests

Change-Id: Ie0dc0dbbfcc13e038352ecf77e2ad28bd41907d9
Reviewed-on: https://chromium-review.googlesource.com/198045
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/buffet.gyp b/buffet/buffet.gyp
index b94677c..ed036bd 100644
--- a/buffet/buffet.gyp
+++ b/buffet/buffet.gyp
@@ -41,6 +41,7 @@
         'async_event_sequencer.cc',
         'manager.cc',
         'mime_utils.cc',
+        'storage_impls.cc',
         'string_utils.cc',
         'url_utils.cc'
       ],