Allocator shim skeleton + Linux impl behind a build flag

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index ce8089159ad067f707944dd209ec98bd9426296d..36dd19e654ddda7aa775a8800c1476e43f295a4f 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -18,6 +18,7 @@
 # huge sequence of random-looking conditionals.
 
 import("//build/buildflag_header.gni")
+import("//build/config/allocator.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/config/sysroot.gni")
@@ -970,6 +971,7 @@ component("base") {
 
   deps = [
     "//base/allocator",
+    "//base/allocator:features",
     "//base/third_party/dynamic_annotations",
     "//third_party/modp_b64",
   ]
@@ -986,6 +988,12 @@ component("base") {
     libs = [ "atomic" ]
   }
 
+  if (use_experimental_allocator_shim) {
+    # The allocator shim is part of the base API. This is to allow clients of
+    # base should to install hooks into the allocator path.
+    public_deps += [ "//base/allocator:unified_allocator_shim" ]
+  }
+
   # Allow more direct string conversions on platforms with native utf8
   # strings
   if (is_mac || is_ios || is_chromeos) {
@@ -1887,6 +1895,10 @@ test("base_unittests") {
     deps += [ ":base_profiler_test_support_library" ]
   }
 
+  if (use_experimental_allocator_shim) {
+    sources += [ "allocator/allocator_shim_unittest.cc" ]
+  }
+
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]