IDL compiler: [ImplementedAs] for [InitializedByEventConstructor]

One use:
core/events/KeyboardEvent.idl
[ImplementedAs=location, DeprecateAs=KeyboardEventKeyLocation, InitializedByEventConstructor] readonly attribute unsigned long keyLocation;

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/102143003

git-svn-id: svn://svn.chromium.org/blink/trunk@163498 bbb929c8-8fbe-4397-9dbb-9b2b20218538