Update class comments for blink::InternalPopupMenu and blink::ExternalPopupMenu.

This CL has no behavior changes.

NOTRY=true

Change-Id: Ic62d65a5574e79120e3a2ac69cd5f0871978c613
Reviewed-on: https://chromium-review.googlesource.com/597034Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491312}

third_party/WebKit/Source/core/html/forms/ExternalPopupMenu.h

@@ -49,8 +49,8 @@ class WebMouseEvent;
 class WebView;
 struct WebPopupMenuInfo;
 
-// The ExternalPopupMenu connects the actual implementation of the popup menu
-// to the WebCore popup menu.
+// The ExternalPopupMenu is a PopupMenu implementation for macOS and Android.
+// It uses a OS-native menu implementation.
 class CORE_EXPORT ExternalPopupMenu final : NON_EXPORTED_BASE(public PopupMenu),
                                             public WebExternalPopupMenuClient {
  public:

third_party/WebKit/Source/core/html/forms/InternalPopupMenu.h

@@ -19,6 +19,8 @@ class HTMLOptGroupElement;
 class HTMLOptionElement;
 class HTMLSelectElement;
 
+// InternalPopupMenu is a PopupMenu implementation for platforms other than
+// macOS and Android. The UI is built with an HTML page inside a PagePopup.
 class CORE_EXPORT InternalPopupMenu final : NON_EXPORTED_BASE(public PopupMenu),
                                             public PagePopupClient {
  public: