Mojo: Require our public options structs to be aligned like int64_t's.

mojo/public/c/system/buffer.h

Index: mojo/public/c/system/buffer.h
diff --git a/mojo/public/c/system/buffer.h b/mojo/public/c/system/buffer.h
index 360832150f4adc947a892c1e094b3a7c05e87fb2..30cf00fa206b03b3e72498a9999312cae1152db6 100644
--- a/mojo/public/c/system/buffer.h
+++ b/mojo/public/c/system/buffer.h
@@ -38,7 +38,7 @@ const MojoCreateSharedBufferOptionsFlags
     ((MojoCreateSharedBufferOptionsFlags) 0)
 #endif
 
-struct MojoCreateSharedBufferOptions {
+struct MOJO_ALIGNAS(MOJO_ALIGNOF(int64_t)) MojoCreateSharedBufferOptions {
   uint32_t struct_size;
   MojoCreateSharedBufferOptionsFlags flags;
 };