Default GR_GL_FUNCTION_TYPE to __stdcall on Windows.

Why is this configurable if we can't work without it?

Just to confirm, this is a Windows constraint, not an MSVC constraint, right?
Clang on Windows also needs __stdcall?

BUG=skia:5617
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2233883002

Review-Url: https://codereview.chromium.org/2233883002
3 files changed