Implement an API to allow passing an "ap" parameter to component updates.

components/component_updater/default_component_installer.h

Index: components/component_updater/default_component_installer.h
diff --git a/components/component_updater/default_component_installer.h b/components/component_updater/default_component_installer.h
index 9de80d6589dc76f566708ab166d7519e08f2c7df..f445fe656d3c661e8d3065fca47058c7699f494f 100644
--- a/components/component_updater/default_component_installer.h
+++ b/components/component_updater/default_component_installer.h
@@ -83,6 +83,12 @@ class ComponentInstallerTraits {
 
   // Returns the human-readable name of the component.
   virtual std::string GetName() const = 0;
+
+  // Returns the additional parameters to be used in the update checks for
+  // this component. A compatible server may use this attribute to negotiate
+  // special update rules when issuing an update response.
+  // The current implementation restricts ap to ^([-+_=a-zA-Z0-9]{0,256})$
+  virtual std::string GetAp() const = 0;
 };
 
 // A DefaultComponentInstaller is intended to be final, and not derived from.