Use start/end for updating selection after text node modification

Before this patch, we update selection after text node modification by
setting relocated base/extent with assumption of base/extent equal to start/end.
However, this assumption is false, when we modify selection by
|VisbileSelection::expandUsingGranularity|, which is used for double clicking
to select word and menu commands, "SelectWord", "SelectLine" and so on.

This patch changes to use start/end positions to update selection after text
node modification. Because, start/end positions are standard selection API
exposed as {anchor,focus}{Node,Offset}, respect to selection direction,
and base/extent are Blink specific selection API.

# Test Updates
- fast/events/updateLayoutForHitTest.html
New behavior sets selection to word "Project" by triple click.

BUG=358947
TEST=LayoutTests/editing/selection/doubleclick-with-split-text.html

Review URL: https://codereview.chromium.org/432633002

git-svn-id: svn://svn.chromium.org/blink/trunk@179392 bbb929c8-8fbe-4397-9dbb-9b2b20218538