Rename PostMessageToMainFrame to PostWebMessage

Bug: 20557074

There is no need to keep the name PostMessageToMainFrame since
we may choose to not implement the special condition to posting
to a named subframe. And if we want, we can do this by overloading
the function.

Change-Id: I9896ad479e1c30cda500352cfdb1b7d336383568
3 files changed