Add LIBMTP_Get_File_Chunk() to read an arbitrary amount of data into memory. The existing APIs only allow reading the entire file with a progress indicator callback.

BUG=160860
Review URL: https://codereview.chromium.org/11312222

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libmtp@167836 4ff67af0-8c30-449e-8e8b-ad334ec8d88c