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

extensions/renderer/module_system.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "extensions/renderer/module_system.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/logging.h"
 #include "base/macros.h"
@@ skipping 151 matching lines @@
       base::Bind(&ModuleSystem::RequireAsync, base::Unretained(this)));
   RouteFunction("privates",
                 base::Bind(&ModuleSystem::Private, base::Unretained(this)));
 
   v8::Local<v8::Object> global(context->v8_context()->Global());
   v8::Isolate* isolate = context->isolate();
   SetPrivate(global, kModulesField, v8::Object::New(isolate));
   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());
   }
 }
 
 ModuleSystem::~ModuleSystem() {
 }
 
 void ModuleSystem::Invalidate() {
   // Clear the module system properties from the global context. It's polite,
@@ skipping 574 matching lines @@
 
 void ModuleSystem::ClobberExistingNativeHandler(const std::string& name) {
   NativeHandlerMap::iterator existing_handler = native_handler_map_.find(name);
   if (existing_handler != native_handler_map_.end()) {
     clobbered_native_handlers_.push_back(std::move(existing_handler->second));
     native_handler_map_.erase(existing_handler);
   }
 }
 
 }  // namespace extensions