GN: Update heap stubs for android

On non-intel (aka non x86/x64) we can compile the assembly stubs as a
source_set and not use yasm.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/541933003

git-svn-id: svn://svn.chromium.org/blink/trunk@181529 bbb929c8-8fbe-4397-9dbb-9b2b20218538