WebLocalFrameImpl::selectWordAroundPosition should select a word before space.

If you have a "foo| ", WebLocalFrameImpl::selectWordAroundPosition
 selects space after the caret.
This is from the Selection::expandUsingGranularity behavior.
It is used widely including double-click word selection on desktop.
Thus we can not change the behavior.

I changed the implementation to use VisibleUnit functions directly.
New implementation is that we try to find a word on right side and left side.
In addition, move implement from WebLocalFrameImpl to FrameSelection.

BUG=397696

Review URL: https://codereview.chromium.org/675413003

git-svn-id: svn://svn.chromium.org/blink/trunk@185359 bbb929c8-8fbe-4397-9dbb-9b2b20218538