Only inject mojo builtins to contexts with the mojoPrivate API feature.

extensions/renderer/module_system.cc

Index: extensions/renderer/module_system.cc
diff --git a/extensions/renderer/module_system.cc b/extensions/renderer/module_system.cc
index f3fdf5d3c1704eb7bf6ba387d9f2b23945755920..8104a02f990825956faf3c7837053b1e241e1f9b 100644
--- a/extensions/renderer/module_system.cc
+++ b/extensions/renderer/module_system.cc
@@ -169,12 +169,9 @@ ModuleSystem::ModuleSystem(ScriptContext* context, const SourceMap* source_map)
   SetPrivate(global, kModuleSystem, v8::External::New(isolate, this));
 
   gin::ModuleRegistry::From(context->v8_context())->AddObserver(this);
-  // TODO(devlin): We really shouldn't be injecting mojo into every blessed
-  // extension context - it's wasteful. But it's better than injecting into
-  // every frame (previous behavior) so start with this while we investigate
-  // further. See crbug.com/636655.
   if (context_->GetRenderFrame() &&
-      context_->context_type() == Feature::BLESSED_EXTENSION_CONTEXT) {
+      context_->context_type() == Feature::BLESSED_EXTENSION_CONTEXT &&
+      context_->GetAvailability("mojoPrivate").is_available()) {

[Devlin, 2016/10/12 14:58:09]

This doesn't seem to be quite the right relationship - mime types handler uses
the mojo builtins, but not mojo private.  I think instead of coupling the two
features, I'd rather just have a list of features that use mojo builtins and
check for the presence of any of them (and add a comment requesting it remain
small).

[Sam McNally, 2016/10/13 07:13:08]

Done.

mojoPrivate does need to make the builtins available; it doesn't use them
directly but instead exposes them to its user.

     context_->GetRenderFrame()->EnsureMojoBuiltinsAreAvailable(
         context->isolate(), context->v8_context());
   }