make base::SharedMemoryHandle a class on POSIX.

base/memory/discardable_shared_memory.cc

Index: base/memory/discardable_shared_memory.cc
diff --git a/base/memory/discardable_shared_memory.cc b/base/memory/discardable_shared_memory.cc
index 99d2991308a3cd503543f5c6da2cbcc6ef1765e1..89d67af827e4adf82aa1d5089e79982dd369d8fc 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -224,9 +224,10 @@ DiscardableSharedMemory::LockResult DiscardableSharedMemory::Lock(
 // Pin pages if supported.
 #if defined(OS_ANDROID)
   SharedMemoryHandle handle = shared_memory_.handle();
-  if (SharedMemory::IsHandleValid(handle)) {
-    if (ashmem_pin_region(
-            handle.fd, AlignToPageSize(sizeof(SharedState)) + offset, length)) {
+  if (handle.IsValid()) {
+    if (ashmem_pin_region(handle.GetHandle(),
+                          AlignToPageSize(sizeof(SharedState)) + offset,
+                          length)) {
       return PURGED;
     }
   }
@@ -251,9 +252,10 @@ void DiscardableSharedMemory::Unlock(size_t offset, size_t length) {
 // Unpin pages if supported.
 #if defined(OS_ANDROID)
   SharedMemoryHandle handle = shared_memory_.handle();
-  if (SharedMemory::IsHandleValid(handle)) {
-    if (ashmem_unpin_region(
-            handle.fd, AlignToPageSize(sizeof(SharedState)) + offset, length)) {
+  if (handle.IsValid()) {
+    if (ashmem_unpin_region(handle.GetHandle(),
+                            AlignToPageSize(sizeof(SharedState)) + offset,
+                            length)) {
       DPLOG(ERROR) << "ashmem_unpin_region() failed";
     }
   }