indexeddb: Removed use of dangling ptr in writeBlobToFileOnIOThread.

LocalWriteClosure only had a raw pointer to a ChainedBlobWriter which
was held by a scoped_refptr in IndexedDBBackingStore::Transaction. If
the transaction was deleted before writeBlobToFileOnIOThread was called
then a crash would ensue. Converted raw ptr to scoped_refptr prevents this.

BUG=395650

Review URL: https://codereview.chromium.org/417573004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285428 0039d316-1c4b-4281-b951-d872f2087c98
10 files changed