Use current selection when tab navigation.

This CL lets FrameSelection::RevealSelection()
 use VisibleUnits::ComputeTextRect() to compute current selection range
 instead of LayoutSelection::Bounds() of last selection range.

VisibleUnits.cpp::ComputeTextRect():
 templatize to define flat tree version.

BUG=712986

Review-Url: https://codereview.chromium.org/2894803002
Cr-Commit-Position: refs/heads/master@{#473546}
5 files changed