Add class comment to ExtensionsMenuButton.

Bug: 943702
Change-Id: Ib7a8fc70ffeabcf62057d82ce0fe34a9c10f8985
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2118896Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753725}

chrome/browser/ui/views/extensions/extensions_menu_button.h

@@ -20,6 +20,9 @@ namespace views {
 class Button;
 }  // namespace views
 
+// ExtensionsMenuButton is the single extension action button within a row in
+// the extensions menu. This includes the extension icon and name and triggers
+// the extension action.
 class ExtensionsMenuButton : public views::LabelButton,
                              public views::ButtonListener,
                              public ToolbarActionViewDelegateViews {