GitLab

* Define GDataDB interface with methods to Put, Get and Delete. Also define a path-based iterator.
* GDataLevelDB implements GDataDB using leveldb. 
* Add methods SerializeToString and FromProtoString to serialize GDataEntry to strings and vice versa.
* GDataDBTests test Put, Get, Delete for files and directories.
* Iterator tests in GDataDBTests.
* GDataDBFactory class to create GDataLevelDB instance.
* Update deps to allow leveldatabase.

TODO:
* There is no integration with GDataRootDirectory/GDataFileSystem yet.

BUG=chromium-os:29232
TEST=unittests pass.
Review URL: https://chromiumcodereview.appspot.com/10210012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@133833 0039d316-1c4b-4281-b951-d872f2087c98