Use explicitly-sized enums in GrGLProgramDesc::KeyHeader

Uses enums explicitly sized to 8 bits in GrGLProgramDesc::KeyHeader,
instead of storing them as uint8_t values. This avoids the need to
static_cast them.

R=bsalomon@google.com

Author: cdalton@nvidia.com

Review URL: https://codereview.chromium.org/23875048

git-svn-id: http://skia.googlecode.com/svn/trunk@11560 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed