fuchsia: Implement base::SharedMemory::GetHandleLimit()

There's no documented handle limit on Fuchsia, and it looks like this
function is used for questionable purposes anyway, so just return
::max().

Bug: 743296
Change-Id: I4e85f31eb519d5dc2d96882d793b9e2c4afc7851
Reviewed-on: https://chromium-review.googlesource.com/597008
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491387}

base/memory/shared_memory_fuchsia.cc

@@ -4,6 +4,8 @@
 
 #include "base/memory/shared_memory.h"
 
+#include <limits>
+
 #include <magenta/process.h>
 #include <magenta/rights.h>
 #include <magenta/syscalls.h>
@@ -36,6 +38,12 @@ void SharedMemory::CloseHandle(const SharedMemoryHandle& handle) {
   handle.Close();
 }
 
+// static
+size_t SharedMemory::GetHandleLimit() {
+  // No documented limit, currently.
+  return std::numeric_limits<size_t>::max();
+}
+
 bool SharedMemory::CreateAndMapAnonymous(size_t size) {
   return CreateAnonymous(size) && Map(size);
 }