add missing header in indexed_db_backing_store.cc

This file uses IsStringASCII but didn't explicitly include the relevant header file.

BUG=259043

Review URL: https://chromiumcodereview.appspot.com/18384006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@211217 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed