Allocator shim skeleton + Linux impl behind a build flag

build/config/allocator.gni

Index: build/config/allocator.gni
diff --git a/build/config/allocator.gni b/build/config/allocator.gni
index 2c908fea6791603a79cdc4e75d2d93b9bee7a07f..eb764ded5747260a66a3050d3d5b412b97c84294 100644
--- a/build/config/allocator.gni
+++ b/build/config/allocator.gni
@@ -24,5 +24,5 @@ declare_args() {
 assert(use_allocator == "none" || use_allocator == "tcmalloc")
 
 assert(!is_win || use_allocator == "none", "Tcmalloc doesn't work on Windows.")
-assert(!use_experimental_allocator_shim || is_linux,
+assert(!use_experimental_allocator_shim || is_linux || is_nacl,

[Nico, 2016/03/08 03:08:42]

why is this needed?

[Primiano Tucci (use gerrit), 2016/03/08 20:54:05]

Oh that didn't really work as expected, removing it.

The full story is that nacl is special in ways I often don't understand.
My plan for flipping the flag in the next CL, is to enable only for non-nacl as
I don't see a compelling use case to have the shim on nacl (unless I am missing
something).

What I get when building with nacl and the shim is:
../../base/allocator/allocator_shim_override_cpp_symbols.h:20:25: error:
function previously declared with an explicit exception specification redeclared
with an implicit exception specification
[-Werror,-Wimplicit-exception-spec-mismatch]
SHIM_ALWAYS_EXPORT void operator delete(void* p) __THROW
                        ^
/s/chrome-shim/src/native_client/toolchain/linux_x86/pnacl_newlib/le32-nacl/include/c++/v1/new:133:30:
note: previous declaration is here
_LIBCPP_NEW_DELETE_VIS void  operator delete(void* __p) _NOEXCEPT;
                             ^

So that thing was really a note-to-self to rule out nacl from the flag. But it
seems that I get if I forget that error is enough of a reminder.

        "use_experimental_allocator_shim supported only on Linux")