DevTools: add prerequirements check to optimize_png_pmages.py

BUG=377780
NOTRY=true

Review URL: https://codereview.chromium.org/302603010

git-svn-id: svn://svn.chromium.org/blink/trunk@175337 bbb929c8-8fbe-4397-9dbb-9b2b20218538