Remove remaining traces of ServiceWorkerMessageEvent

The ServiceWorkerMessageEvent.* files were already removed from BUILD.gn:
https://codereview.chromium.org/2466513002

This inlines the templatized helpers in V8ServiceWorkerMessageEventInternal.h
into non-templated code in V8ExtendableMessageEventCustom.cpp, which is now the
only place the helpers are used.

BUG=659074

Review-Url: https://codereview.chromium.org/2837353003
Cr-Commit-Position: refs/heads/master@{#467291}
6 files changed