GrTessellator: don't reuse the previous vertex buffer on a cache miss.

Just let the cache give us the correct entry for the given size.
This simplifies the code (no mysterious non-const ref member).
I don't see any performance effect from this change, but will watch
the bots closely.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1800623002

Review URL: https://codereview.chromium.org/1800623002
1 file changed