Use alignas to force alignment.

Using alignas reduces code and platform specific macros.

Use alignas instead of std::aligned_storage because it is unimplemneted
in MSVC 2015.

Here is the bug from MS:
https://connect.microsoft.com/VisualStudio/feedback/details/1559873/std-aligned-storage-cannot-align-type-with-16-byte

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2468243002
TBR=reed@google.com

Review-Url: https://codereview.chromium.org/2473143002
3 files changed