Drop [LegacyImplementedInBaseClass] attribute from WindowTimers

Drop [LegacyImplementedInBaseClass] IDL extended attribute from the
WindowTimers interface and move the code to a new DOMWindowTimers
file to reduce code duplication.

There is still some duplication in the custom bindings code but this
will be addressed in a later patch to limit patch size.

R=arv@chromium.org, haraken@chromium.org

Review URL: https://codereview.chromium.org/19801002

git-svn-id: svn://svn.chromium.org/blink/trunk@154579 bbb929c8-8fbe-4397-9dbb-9b2b20218538