Add a stub __cxa_demangle to disable LLVM's demangler.

base/android/cxa_demangle_stub.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <unistd.h>
+
+// LLVM's demangler is large, and we have no need of it.  Overriding it with
+// our own stub version here stops a lot of code being pulled in from libc++.
+// More here:
+//   https://llvm.org/svn/llvm-project/libcxxabi/trunk/src/cxa_demangle.cpp
+extern "C" char* __cxa_demangle(const char* mangled_name,
+                                char* buf,
+                                size_t* n,
+                                int* status) {
+  static const int kMemoryAllocFailure = -1;  // LLVM's memory_alloc_failure.

[rmcilroy, 2015/06/26 12:43:49]

Can we just do a CHECK(false) here instead? This would make it more obvious if
someone were to introduce a dependency on cxa_demangler as to what was going
wrong. 

Also, should we do this for all Posix platforms that pull in libc++,  or are
there requirements on cxa_demangler on other platforms?

[simonb (inactive), 2015/06/26 15:00:09]

I'm pretty sure we don't want to CHECK(false) here.  If LLVM or something else
calls this we don't want to crash, just indicate that demangling is effectively
disabled.  LLVM already has a dependency on it, which is why the code is drawn
in from libc++.  What we want to do is lighten the implementation while leaving
it still functional.

See the parallel PNaCl change:
https://codereview.chromium.org/102883003
Our requirement is to keep libchrome slim on Android.  Extending it to other
posix may be possible/desirable, but these aren't environments we generally work
in.

[rmcilroy, 2015/06/29 09:24:18]

Ok sounds good, thanks for the explanation.
Fair enough.

+  if (status)
+    *status = kMemoryAllocFailure;
+  return nullptr;
+}