port most SkAtomic<T> to std::atomic<T>

The only SkAtomic<T> left are the ones overriding the default memory
order in SkPath.  I think I'd like to try switching them to std::atomic
too, but in another CL.

Trim unused APIs in SkAtomics.h.

Change-Id: Ia1c283355902ccb8fcdad70cdf27bb577e8ca407
Reviewed-on: https://skia-review.googlesource.com/146529
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/src/core/SkSharedMutex.h b/src/core/SkSharedMutex.h
index 33f1f10..1177844 100644
--- a/src/core/SkSharedMutex.h
+++ b/src/core/SkSharedMutex.h
@@ -8,10 +8,10 @@
 #ifndef SkSharedLock_DEFINED
 #define SkSharedLock_DEFINED
 
-#include "SkAtomics.h"
 #include "SkMacros.h"
 #include "SkSemaphore.h"
 #include "SkTypes.h"
+#include <atomic>
 
 #ifdef SK_DEBUG
     #include "SkMutex.h"
@@ -59,9 +59,9 @@
     SkSemaphore fSharedQueue[2];
     SkSemaphore fExclusiveQueue;
 #else
-    SkAtomic<int32_t> fQueueCounts;
-    SkSemaphore       fSharedQueue;
-    SkSemaphore       fExclusiveQueue;
+    std::atomic<int32_t> fQueueCounts;
+    SkSemaphore          fSharedQueue;
+    SkSemaphore          fExclusiveQueue;
 #endif  // SK_DEBUG
 };