document the non-ART-related reasons for execmem

Change-Id: I455fe33345dd1ae8dc49cb7b70cbf1e7c1b3e271

app.te

@@ -7,8 +7,9 @@
 ### zygote spawned apps should be added here.
 ###
 
-# Dalvik Compiler JIT Mapping.
+# WebView and other application-specific JIT compilers
 allow appdomain self:process execmem;
+
 allow appdomain ashmem_device:chr_file execute;
 
 # Receive and use open file descriptors inherited from zygote.

mediaserver.te

@@ -14,7 +14,9 @@ binder_call(mediaserver, binderservicedomain)
 binder_call(mediaserver, appdomain)
 binder_service(mediaserver)
 
+# Required by Widevine DRM (b/22990512)
 allow mediaserver self:process execmem;
+
 allow mediaserver kernel:system module_request;
 allow mediaserver media_data_file:dir create_dir_perms;
 allow mediaserver media_data_file:file create_file_perms;