localStorage: Flushing unwritten data in Android's ChromiumApplication.

This adds a new StoragePartition::Flush method to flush all unwritten data
for the StoragePartition. At present this only flushes the
DOMStorageContextImpl - not sure if other storage systems require flushing.

Also, included a few headers (as suggested by git cl lint), and fixed a few
typos.

BUG=479767

Review URL: https://codereview.chromium.org/1126733002

Cr-Commit-Position: refs/heads/master@{#328856}
diff --git a/content/browser/storage_partition_impl.h b/content/browser/storage_partition_impl.h
index 3028b61..0aeb104 100644
--- a/content/browser/storage_partition_impl.h
+++ b/content/browser/storage_partition_impl.h
@@ -71,6 +71,8 @@
                  const base::Time end,
                  const base::Closure& callback) override;
 
+  void Flush() override;
+
   WebRTCIdentityStore* GetWebRTCIdentityStore();
 
   // Can return nullptr while |this| is being destroyed.