Chrome-side of fixes for content-script messaging.

Chrome-side of fixes for content-script messaging.

This change adds registration of content scripts, parented to a frame's
context.  When a frame's context goes away, we unregister it and any content
script contexts for it.

There's a corresponding webkit change that lets us know when a content script
context is created.  Filed upstream as
https://bugs.webkit.org/show_bug.cgi?id=27104.

BUG=16228
TEST=install an extension with a content script that communicates with a parent
process.  Messages should be sendable both ways.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=20677

[Aaron Boodman, 2009-07-10 01:18:16]

lgtm