Add constructor for CPDF_ColorSpace.

Prevents sub-classes from tweaking parent class instance vars.

Also "fix" https://code.google.com/p/pdfium/issues/detail?id=170, assuming 0 is a good value.

BUG=pdfium:170
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1185903002.