allocator: add use_experimental_allocator_shim build flag

third_party/tcmalloc/chromium/src/libc_override.h

Index: third_party/tcmalloc/chromium/src/libc_override.h
diff --git a/third_party/tcmalloc/chromium/src/libc_override.h b/third_party/tcmalloc/chromium/src/libc_override.h
index 089084a2e4988d34fd0723f4d651f194e264a2c4..de18abb8db33378b576d5583c4cfe069b061d608 100644
--- a/third_party/tcmalloc/chromium/src/libc_override.h
+++ b/third_party/tcmalloc/chromium/src/libc_override.h
@@ -59,12 +59,16 @@
 
 static void ReplaceSystemAlloc();  // defined in the .h files below
 
+#if defined(TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC)
+
+static void ReplaceSystemAlloc() {}

[Will Harris, 2016/02/08 19:23:55]

add some comment here. it's a bit subtle what this is doing.

[Primiano Tucci (use gerrit), 2016/02/09 11:38:53]

Done.

+
 // 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(); }