tcmalloc: allocator: add use_experimental_allocator_shim build flag

Original CL:

- https://codereview.chromium.org/1678893002

  allocator: add use_experimental_allocator_shim build flag

  This CL introduces a new GYP/GN build flag
  use_experimental_allocator_shim (default: false).  After this CL, the
  only side-effect of that flag is that tcmalloc does not override
  anymore the libc symbols (malloc, new etc.).  This is in preparation
  of a unified shim layer, which will come separately in upcoming CLs,
  which will take care of overriding those symbols in Chrome.

  Design doc: http://bit.ly/allocator-shim

  BUG=550886
  Committed: https://crrev.com/3217078aa86523fb341e0cf2b0c49d55434b534a
  Cr-Commit-Position: refs/heads/master@{#374449}

BUG=724399,b:70905156

Change-Id: Iaa205ec87dc160a2e738cc0d9adfe3c66acb12d9
Reviewed-on: https://chromium-review.googlesource.com/1130785
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580319}

third_party/tcmalloc/chromium/src/libc_override.h

@@ -68,17 +68,31 @@
 
 static void ReplaceSystemAlloc();  // defined in the .h files below
 
+#if defined(TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC)
+// TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC has the following semantic:
+//  - tcmalloc with all its tc_* (tc_malloc, tc_free) symbols is being built
+//    and linked as usual.
+//  - the default system allocator symbols (malloc, free, operator new) are NOT
+//    overridden. The embedded must take care of routing them to tc_* symbols.
+// This no-op #if block effectively prevents the inclusion of the
+// libc_override_* headers below.
+static void ReplaceSystemAlloc() {}
+
 // For windows, there are two ways to get tcmalloc.  If we're
 // patching, then src/windows/patch_function.cc will do the necessary
 // overriding here.  Otherwise, we doing the 'redefine' trick, where
 // we remove malloc/new/etc from mscvcrt.dll, and just need to define
 // them now.
-#if defined(_WIN32) && defined(WIN32_DO_PATCHING)
+#elif defined(_WIN32) && defined(WIN32_DO_PATCHING)
 void PatchWindowsFunctions();   // in src/windows/patch_function.cc
 static void ReplaceSystemAlloc() { PatchWindowsFunctions(); }
 
 #elif defined(_WIN32) && !defined(WIN32_DO_PATCHING)
-#include "libc_override_redefine.h"
+// "libc_override_redefine.h" is included in the original gperftools.  But,
+// we define allocator functions in Chromium's base/allocator/allocator_shim.cc
+// on Windows.  We don't include libc_override_redefine.h here.
+// ReplaceSystemAlloc() is defined here instead.
+static void ReplaceSystemAlloc() {}
 
 #elif defined(__APPLE__)
 #include "libc_override_osx.h"