[mojo-edk] Expose notification source to MojoWatch callbacks

mojo/edk/system/request_context.h

Index: mojo/edk/system/request_context.h
diff --git a/mojo/edk/system/request_context.h b/mojo/edk/system/request_context.h
index 56b849870e33ba07048d5f450e911880be51b045..394e455327499b56bb3ee0252dd7938a8ff6945a 100644
--- a/mojo/edk/system/request_context.h
+++ b/mojo/edk/system/request_context.h
@@ -8,6 +8,7 @@
 #include "base/containers/stack_container.h"
 #include "base/macros.h"
 #include "mojo/edk/system/handle_signals_state.h"
+#include "mojo/edk/system/system_impl_export.h"
 #include "mojo/edk/system/watcher.h"
 
 namespace base {
@@ -27,14 +28,25 @@ namespace edk {
 // for any reason. Therefore it is important to always use
 // |RequestContext::current()| rather than referring to any local instance
 // directly.
-class RequestContext {
+class MOJO_SYSTEM_IMPL_EXPORT RequestContext {
  public:
+  // Identifies the source of the current stack frame's RequestContext.
+  enum class Source {
+    LOCAL_API_CALL,
+    SYSTEM,
+  };
+
+  // Constructs a RequestContext with a LOCAL_API_CALL Source.
   RequestContext();
+
+  explicit RequestContext(Source source);
   ~RequestContext();
 
   // Returns the current thread-local RequestContext.
   static RequestContext* current();
 
+  Source source() const { return source_; }
+
   // Adds a finalizer to this RequestContext corresponding to a watch callback
   // which should be triggered in response to some handle state change. If
   // the Watcher hasn't been cancelled by the time this RequestContext is
@@ -74,6 +86,8 @@ class RequestContext {
   using WatchCancelFinalizerList =
       base::StackVector<scoped_refptr<Watcher>, kStaticWatchFinalizersCapacity>;
 
+  const Source source_;
+
   WatchNotifyFinalizerList watch_notify_finalizers_;
   WatchCancelFinalizerList watch_cancel_finalizers_;