[NOT FOR REVIEW] Basic allocator shim for macOS.

base/allocator/allocator_shim.cc

Index: base/allocator/allocator_shim.cc
diff --git a/base/allocator/allocator_shim.cc b/base/allocator/allocator_shim.cc
index 95480ea4b6b510f52a244d529da7a16385c9d035..830348b6eaa5707136c2ef9c35456fd8ef025020 100644
--- a/base/allocator/allocator_shim.cc
+++ b/base/allocator/allocator_shim.cc
@@ -243,6 +243,13 @@ void ShimFree(void* address) {
   return chain_head->free_function(chain_head, address);
 }
 
+size_t ShimGetSizeEstimate(const void* address) {
+  const allocator::AllocatorDispatch* const chain_head = GetChainHead();
+  // TODO: Fix get_size_estimate() to take 'const void*'.
+  return chain_head->get_size_estimate_function(
+      chain_head, const_cast<void*>(address));
+}
+
 }  // extern "C"
 
 #if !defined(OS_WIN)
@@ -259,6 +266,8 @@ void ShimFree(void* address) {
 #elif defined(OS_WIN)
 // On Windows we use plain link-time overriding of the CRT symbols.
 #include "base/allocator/allocator_shim_override_ucrt_symbols_win.h"
+#elif defined(OS_MACOSX)
+#include "base/allocator/allocator_shim_override_mac_symbols.h"
 #else
 #include "base/allocator/allocator_shim_override_libc_symbols.h"
 #endif