Add instructions for building with MSAN

Move static functions only used by GPU into if SK_SUPPORT_GPU guard.
This enables building MSAN, which seems to default to not supporting
GPU.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1908493002
DOCS_PREVIEW= https://skia.org/?cl=1908493002

Review URL: https://codereview.chromium.org/1908493002
2 files changed