Fix wrong cwd in getChromiumSrcDir().

Working directory can be wrong when calling getChromiumSrcDir.
On Windows cwd is inside Chrome source directory when it called
from repack_locales.py that causes git config to return empty
value. This causes the function to fall back to snapshot directory
even if upstream is used.

Changing cwd to qtwebengine_root, running git config and restoring
original cwd resolves the problem.

Change-Id: I1bef84ce791f63d3c9e7d6c9506e99c0c53cfc3e
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>