[WK2] revised a callflow of ewkViewFrameRendered after nonempty layout finished.

[WK2] revised a callflow of ewkViewFrameRendered after nonempty layout finished.

[Title] [WK2] revised a callflow of ewkViewFrameRendered after nonempty layout finished.
[Issue#] N/A
[Problem] when resizing a web-provider box, webview is bliking.
[Cause] the client does not know when the actual rendering is started.
[Solution] Send "frame,rendered" message to the client
           after didFirstVisuallyNonEmptyLayoutForMainFrame is called.
           the message sequence is nonemptyLayout -> composite -> frame,rendered -> buffer update.

Change-Id: I41c1b1e19e0afd99b8e928c4f8b8784a1cf9b187