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,Test-iOS-Clang-iPad4-GPU-SGX554-Arm7-Release-Trybot

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

Committed: https://skia.googlesource.com/skia/+/70e8dfca4a7f5bce97b8021a6e378c4828b09c8c

Committed: https://skia.googlesource.com/skia/+/dadfc245cc9a0279ff7b73da3344f2ca5d139907
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1436033003

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