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..e5fb1e9c542e23ae4f10e02f9b472b166920cf74 100644
--- a/extensions/renderer/module_system.cc
+++ b/extensions/renderer/module_system.cc
@@ -111,6 +111,22 @@ void SetExportsProperty(
     LOG(ERROR) << "Failed to set private property on the export.";
 }
 
+bool ContextNeedsMojoBindings(ScriptContext* context) {
+  // Mojo is only used from JS by some APIs so a context only needs the mojo
+  // bindings at least one is available.

[Devlin, 2016/10/13 14:44:14]

nit: "if at least one is available"? (missing "if")

[Sam McNally, 2016/10/13 22:24:50]

Done.

+  //
+  // Prefer to use Mojo from C++ if possible rather than adding to this list.
+  static const char* const kMojoUsingApis[] = {

[Devlin, 2016/10/13 14:44:14]

nitty nit: since variable names can't express the difference between "mojo-using
apis" (what this means) and "mojo using apis" (apis that are used by mojo),
let's rename this to something like "kApisUsingMojo" or "kApisRequiringMojo" (or
something along those lines).

[Sam McNally, 2016/10/13 22:24:50]

Done.

+      "mimeHandlerPrivate", "mojoPrivate",
+  };
+
+  for (const auto* api : kMojoUsingApis) {
+    if (context->GetAvailability(api).is_available())
+      return true;
+  }
+  return false;
+}
+
 }  // namespace
 
 std::string ModuleSystem::ExceptionHandler::CreateExceptionString(
@@ -169,12 +185,9 @@ 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 &&
+      ContextNeedsMojoBindings(context_)) {
     context_->GetRenderFrame()->EnsureMojoBuiltinsAreAvailable(
         context->isolate(), context->v8_context());
   }