Fix the bug that fixed element has wrong size

Fix the bug that fixed element has wrong size

[Title] Fix the bug that fixed element has wrong size
[Issues] N_SE-52245
[Problem] Fixed position has wrong size after showing IME.
[Cause] While calling WebGraphicsLayer::setSize, if the element has position:fixed property then
        the graphics layer be able to get wrong backing store size because the height of cover rect
        is very small when showing IME.
[Solution] Does not call adjustVisibleRect in case of the fixed element in order to prevent wrong
           backingstore size.

Change-Id: Idf1f909f5d6afe2457ab93ab2919d2a8609aa898