[Extensions] Don't inject mojo bindings for every context

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);
-  if (context_->GetRenderFrame()) {
+  // 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) {

[haraken, 2016/08/18 01:42:59]

Does this mean that we don't inject mojo bindings for the main world (in the
Blink terms)?

[Devlin, 2016/08/18 02:01:36]

This means we only inject bindings for extension frames.  So if you visit a
chrome-extension:// page, that main frame will get bindings, but for the 99.9%
of other traffic, there are no bindings (for the main frame or any js worlds). 
With site isolation, extension iframes are considered blessed extension
contexts, so they will get bindings, but again, that's fairly rare.

     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