Add skia_use_safe_libcxx

Setting this flag to true enables libc++'s safe mode.
This enables things like bounds checking on operator[]
for vector.

For details, see:
https://libcxx.llvm.org/UsingLibcxx.html#enabling-the-safe-libc-mode

Bug: chromium:1432603
Change-Id: Ie1e39b75b88f007175712e0c91c41aa45248aa7e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/717281
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed