Revert 133815 - GDataDB support with leveldb.

Revert 133815 - GDataDB support with leveldb.

* 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.

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

BUG=chromium-os:29232
TEST=unittests pass.
Review URL: https://chromiumcodereview.appspot.com/10168025

TBR=achuith@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=133819