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..55c7bab3f63e5cd0c511084e4d05b3e8c7b241af 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 if at least one is available.
+  //
+  // Prefer to use Mojo from C++ if possible rather than adding to this list.
+  static const char* const kApisRequiringMojo[] = {
+      "mimeHandlerPrivate", "mojoPrivate",
+  };
+
+  for (const auto* api : kApisRequiringMojo) {
+    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());
   }