Accept IContextSource from EditFilterMergedContent hook.

This is required for Translate to work if and when I99a19c93 is merged.
It removes the assumption that EditFilterMergedContent will always
provide an EditPage as its first parameter.

Change-Id: I8b6a75a31606e4693de2ca3ed1f52806d5f92e06
Merge "Accept IContextSource from EditFilterMergedContent hook."