Added CSSPropertyDescriptor.h.tmpl to generate CSSPropertyDescriptor.h.

Added CSSPropertyDescriptor.h.tmpl and modified
make_css_property_apis.py to generate CSSPropertyDescriptor.h. As a
result of this, new methods added to the API no longer need to be added
to the struct in CSSPropertyDescriptor.h - the function pointers in this
file correspond to the methods defined in CSSPropertyAPIMethods.json5.

Gist of the generated code:
https://gist.github.com/anonymous/b315505ebc11cbd8901afd3bb01a620a

Diff of original code vs generated:
https://www.diffchecker.com/qPe6SX85

Original patch:
https://codereview.chromium.org/2684353002

BUG=668012

Review-Url: https://codereview.chromium.org/2706623004
Cr-Commit-Position: refs/heads/master@{#451702}
4 files changed