Delay creation of cpu-side buffer memory until actually needed

IIUC what is going on, this won't really do anything bad but will defer allocation of the cpu-side buffer until it is actually needed.

BUG=635015
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2248283007

Review-Url: https://codereview.chromium.org/2248283007
1 file changed