Make size of hypervisor heap a build parameter.

Bug: 117270899
Change-Id: Ie9fd8db6a795f65637035773cfb43cbf1b89b6ec
diff --git a/build/toolchain/platform.gni b/build/toolchain/platform.gni
index 4bcfdc3..e2fa6a4 100644
--- a/build/toolchain/platform.gni
+++ b/build/toolchain/platform.gni
@@ -20,6 +20,9 @@
   # The architecture of the platform.
   plat_arch = ""
 
+  # The number of pages to allocate for the hypervisor heap.
+  plat_heap_pages = 0
+
   # The maximum number of CPUs available on the platform.
   plat_max_cpus = 0