Make `download_material_design_icons` work in a clean checkout.
Make `download_material_design_icons` work in a clean checkout.

This patch makes the call to rmtree that wipes the already downloaded
icons pass if the directory is not present yet.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1226833009 .

jianglu/mojo,afandria/mojo,jianglu/mojo,chinmaygarde/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,chinmaygarde/mojo,chinmaygarde/mojo,jianglu/mojo,afandria/mojo,chinmaygarde/mojo