SkPath::fFirstDirection: seq-cst -> relaxed

SkPath::fFirstDirection: seq-cst -> relaxed

We landed this originally with lazily-correct sequentially-consistent memory
order.  It turns out that's regressed performance, we think particularly when
recording paths.  We also think there's no need for anything but relaxed memory
order here.

We should see this chart go down if all goes well: https://perf.skia.org/#4329
There are also Chrome performance charts to watch in the linked bug.

BUG=chromium:537700

CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN-Trybot,Test-Ubuntu-GCC-Golo-GPU-GT610-x86_64-Release-TSAN

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

Review URL: https://codereview.chromium.org/1393833003