Added ashmem_get_size_region() function.

This is needed by the MemoryFile changes in
https://android-git.corp.google.com/g/2714
where it is used to find out whether a file descriptor
refers to an ashmem region.
diff --git a/libcutils/ashmem-dev.c b/libcutils/ashmem-dev.c
index 5e158af..8b71f87 100644
--- a/libcutils/ashmem-dev.c
+++ b/libcutils/ashmem-dev.c
@@ -83,3 +83,8 @@
 	struct ashmem_pin pin = { offset, len };
 	return ioctl(fd, ASHMEM_UNPIN, &pin);
 }
+
+int ashmem_get_size_region(int fd)
+{
+  return ioctl(fd, ASHMEM_GET_SIZE, NULL);
+}