Make mojo::system::Core not own the PlatformSupport.

mojo/edk/embedder/embedder_internal.h

Index: mojo/edk/embedder/embedder_internal.h
diff --git a/mojo/edk/embedder/embedder_internal.h b/mojo/edk/embedder/embedder_internal.h
index e9fc4b8866c4ee8d2e6c59ee2833d422cd2368f3..536e0dbe729cee53d1badb105dbdda4ec65d4e01 100644
--- a/mojo/edk/embedder/embedder_internal.h
+++ b/mojo/edk/embedder/embedder_internal.h
@@ -25,6 +25,8 @@ typedef uint64_t ChannelId;
 
 namespace embedder {
 
+class PlatformSupport;
+
 // This is a type that's opaque to users of the embedder API (which only
 // gives/takes |ChannelInfo*|s). We make it a struct to make it
 // template-friendly.
@@ -37,6 +39,9 @@ struct ChannelInfo {
 
 namespace internal {
 
+// Instance of |PlatformSupport| to use.
+extern PlatformSupport* g_platform_support;
+
 // Instance of |Core| used by the system functions (|Mojo...()|).
 extern system::Core* g_core;