Bug 1480732 - make ipc/'s atomicops.h work on aarch64 windows; r=jld

I'm not entirely sure how this works on x86-64 Windows, which also uses
a #define for MemoryBarrier...I think something about intrinsics.

ipc/chromium/src/base/atomicops.h

@@ -98,6 +98,20 @@ Atomic32 Release_CompareAndSwap(volatile Atomic32* ptr,
                                 Atomic32 old_value,
                                 Atomic32 new_value);
 
+// On AArch64 Windows, MemoryBarrier is defined as:
+//
+// #define MemoryBarrier()             __dmb(_ARM_BARRIER_SY)
+//
+// which wreaks havoc with the declaration below.  Capture the definition
+// before undefining the macro.
+#if defined(_M_ARM64) && defined(MemoryBarrier)
+static inline void
+MemoryBarrierARM64()
+{
+  MemoryBarrier();
+}
+#undef MemoryBarrier
+#endif
 void MemoryBarrier();
 void NoBarrier_Store(volatile Atomic32* ptr, Atomic32 value);
 void Acquire_Store(volatile Atomic32* ptr, Atomic32 value);
@@ -136,6 +150,9 @@ Atomic64 Release_Load(volatile const Atomic64* ptr);
 // Include our platform specific implementation.
 #if defined(OS_WIN) && defined(ARCH_CPU_X86_FAMILY)
 #include "base/atomicops_internals_x86_msvc.h"
+#elif defined(OS_WIN) && defined(ARCH_CPU_AARCH64_FAMILY)
+// Works just fine, separate case in case we need to change things.
+#include "base/atomicops_internals_x86_msvc.h"
 #elif defined(OS_MACOSX) && defined(ARCH_CPU_X86_FAMILY)
 #include "base/atomicops_internals_x86_macosx.h"
 #elif defined(COMPILER_GCC) && defined(ARCH_CPU_X86_FAMILY)

ipc/chromium/src/base/atomicops_internals_x86_msvc.h

@@ -45,8 +45,13 @@ inline Atomic32 NoBarrier_AtomicIncrement(volatile Atomic32* ptr,
 }
 
 inline void MemoryBarrier() {
-  // We use MemoryBarrier from WinNT.h
+  // We use MemoryBarrier from WinNT.h, except for AArch64.
+  // See the comment in atomicops.h.
+#if defined(_M_ARM64)
+  MemoryBarrierARM64();
+#else
   ::MemoryBarrier();
+#endif
 }
 
 inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr,