Refactoring: Rename 'innerBlock', which is a shadow element of <input>, to 'editingViewPort.'

The name 'innerBlock' was not helpful because a text field <input> contains
multiple blocks in its shadow. 'innerBlock' is used to clip visible area of the
editing element, and we name it 'editingViewPort.'

BUG=none
TEST=none; no behavior changes.
R=ksakamoto@chromium.org

Review URL: https://codereview.chromium.org/24247002

git-svn-id: svn://svn.chromium.org/blink/trunk@157998 bbb929c8-8fbe-4397-9dbb-9b2b20218538