Allocator shim skeleton + Linux impl behind a build flag

base/base.gyp

Index: base/base.gyp
diff --git a/base/base.gyp b/base/base.gyp
index 2b93cdac4ab5a7f43667d6493d24bf525c6ec1ac..b6398c53252d7970c9141c769046ee7d0494416e 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -22,6 +22,7 @@
       },
       'dependencies': [
         'allocator/allocator.gyp:allocator',
+        'allocator/allocator.gyp:allocator_features',
         'base_debugging_flags#target',
         'base_static',
         'base_build_date#target',
@@ -234,6 +235,9 @@
             'sync_socket_posix.cc',
           ]
         }],
+        ['use_experimental_allocator_shim==1', {
+          'dependencies': [ 'allocator/allocator.gyp:unified_allocator_shim']
+        }],
       ],
       'sources': [
         'auto_reset.h',
@@ -718,6 +722,9 @@
             'third_party/libevent/libevent.gyp:libevent'
           ],
         }],
+        ['use_experimental_allocator_shim==1', {
+          'sources': [ 'allocator/allocator_shim_unittest.cc']
+        }],
       ],  # conditions
       'target_conditions': [
         ['OS == "ios" and _toolset != "host"', {