Start refactoring Windows allocator shim in prep for hooking in generic allocator shim on Windows.

base/allocator/allocator.gyp

Index: base/allocator/allocator.gyp
diff --git a/base/allocator/allocator.gyp b/base/allocator/allocator.gyp
index 674d4d645f79a2b4a7de7d56589c4bacdb479e65..af27c376b4d3e74c911e6076f17c672f76cd0ed9 100644
--- a/base/allocator/allocator.gyp
+++ b/base/allocator/allocator.gyp
@@ -55,6 +55,8 @@
           'sources': [
             'allocator_shim_win.cc',
             'allocator_shim_win.h',
+            'winheap_stubs_win.cc',
+            'winheap_stubs_win.h',
           ],
           'configurations': {
             'Debug_Base': {
@@ -386,6 +388,7 @@
         'buildflag_header_path': 'base/allocator/features.h',
         'buildflag_flags': [
           'USE_EXPERIMENTAL_ALLOCATOR_SHIM=<(use_experimental_allocator_shim)',
+          'ENABLE_WIN_ALLOCATOR_SHIM_TESTS=<(use_experimental_allocator_shim)||<(win_use_allocator_shim)',
         ],
       },
     },  # 'allocator_features' target.