Change export_from_git.py to take foll snapshots from upstream.

The updating procedure for chromium is now as follows:
- update 3rdparty_upstream to the requested state for the snapshot
- initialize 3rdparty submodule by executing
    git submodule init -- 3rdparty
    git submodule update -- 3rdparty
- execute the export_from_git.py script from the QtWebEngine
  top level directory.
- do a "git commit -a" in 3rdparty/ and write an appropriate
  commit message mentioning the exact state from where the snapshot
  was taken from.
- do a "git show" to double check the changes for the snapshot look
  reasonable.
- push the changes to the 3rdparty repository.

Change-Id: I7806d284c9a146f857600cfef0face8331af646a
Reviewed-by: Andras Becsi <andras.becsi@digia.com>