<third_party/WebKit> Prevent a python IOError on Windows due to MAX_PATH

Running python down into the WebKit directory structure and referring
a file relatively to the build directory can easily spill over the
260 char limit on Windows.

Resolve the absolute path using join and normpath to work around the
issue.

Change-Id: I6e89aad542761adcbd821b05f0cc6e9d489dced5
Reviewed-by: Andras Becsi <andras.becsi@digia.com>