skstd -> std for unique_ptr

skstd -> std for unique_ptr

TBR=reed@google.com
No public API changes.

BUG=skia:4564

Committed: https://skia.googlesource.com/skia/+/755c553c17b82bb5de3d9cc8d3b2a866ff9e9e50

CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Mac10.9-Clang-x86_64-Release-CMake-Trybot,Build-Ubuntu-GCC-x86_64-Debug-CrOS_Link-Trybot;client.skia:Perf-Mac10.9-Clang-MacMini6.2-CPU-AVX-x86_64-Release-Trybot

Committed: https://skia.googlesource.com/skia/+/06189155d987db5c7e69015f6ea87c2168d6a065

Review URL: https://codereview.chromium.org/1436033003