Direct call setFixedVisibleContentRect() by WebProcess when viewport is changed

Direct call setFixedVisibleContentRect() by WebProcess when viewport is changed

[Title] Direct call setFixedVisibleContentRect() on WebProcess when viewport is changed
[Issue] WEB-1495, S1-7011
[Problem] Timing dependent different JS window sizes.
[Cause] When viewport is changed,
        visible content rect is not set directly by WebProcess itself.
        So, WebCore returns incorrect visible content rect
        until UIProcess updates WebProcess's visible contentrect.
[Solution] WebProcess calls setFixedVisibleContentRect directly.

Conflicts:

	Source/WebKit2/WebProcess/WebPage/WebPage.cpp

Change-Id: Id70c640a58a7603162f88c422c3267d2b78bfb9d