Make GrTextureStripAtlas flush pending IO on newly acquired texture

GrTextureStripAtlas uses its own lock counts to protect against
overwriting its own earlier writes, but that doesn't protect against
IO that was pending when a texture was first acquired.

BUG=chromium:637678
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2262233002

Review-Url: https://codereview.chromium.org/2262233002
8 files changed