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

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 4665d753f37c579d09e2afa60f64d4df60bb2d1c..8f0fbaf7d90defa1c66ec07a28fdc3d57d2b961f 100644
--- a/extensions/renderer/module_system.cc
+++ b/extensions/renderer/module_system.cc
@@ -169,7 +169,12 @@ ModuleSystem::ModuleSystem(ScriptContext* context, const SourceMap* source_map)
   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());
   }