Issue #618 - (async) Implement async attribute for inline module scripts.

This commit does several things:
- Moves the pref check from ScriptLoader to ns[I]Document so it can be called on
the document.
- Changes the atrribute freezing function to a better name that takes the
document as a parameter.
- Sets the proper async/defer attributes on HTML script elements based on
keywords and whether they are module scripts or not.

dom/html/HTMLScriptElement.cpp

@@ -280,12 +280,20 @@ HTMLScriptElement::GetScriptCharset(nsAString& charset)
 }
 
 void
-HTMLScriptElement::FreezeUriAsyncDefer()
+HTMLScriptElement::FreezeExecutionAttrs(nsIDocument* aOwnerDoc)
 {
   if (mFrozen) {
     return;
   }
 
+  MOZ_ASSERT(!mIsModule && !mAsync && !mDefer && !mExternal);
+
+  // Determine whether this is a classic script or a module script.
+  nsAutoString type;
+  GetScriptType(type);
+  mIsModule = aOwnerDoc->ModuleScriptsEnabled() &&
+              !type.IsEmpty() && type.LowerCaseEqualsASCII("module");
+
   // variation of this code in nsSVGScriptElement - check if changes
   // need to be transfered when modifying.  Note that we don't use GetSrc here
   // because it will return the base URL when the attr value is "".
@@ -300,14 +308,13 @@ HTMLScriptElement::FreezeUriAsyncDefer()
 
     // At this point mUri will be null for invalid URLs.
     mExternal = true;
-
-    bool defer, async;
-    GetAsync(&async);
-    GetDefer(&defer);
-
-    mDefer = !async && defer;
-    mAsync = async;
   }
+  
+  bool defer, async;
+  GetAsync(&async);
+  mAsync = (mExternal || mIsModule) && async;
+  GetDefer(&defer);
+  mDefer = mExternal && !async && defer;
 
   mFrozen = true;
 }