In review: http://crrev.com/2723133004

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/67adef739b13e794850c228f3677dd75614958c1

commit 67adef739b13e794850c228f3677dd75614958c1
Author: yosin <yosin@chromium.org>
Date: Mon Mar 06 05:34:16 2017

Make FrameSeelction::firstRange to return proper range

This patch makes |FrameSeelction::firstRange()| to return proper range instead
of the range specified by |FrameSelection::setSelectedRange()|.

Before this patch, |FrameSelection::firstRange()| returns the range we called
"logical range" introduced by patch[1], which mitigated some situation but it
is incomplete. Patch[2] introduced complete solution and compatible with the
specification[3].

This patch is follow-up of patch[2] to remove "logical range" from Blink since
cached |Range| supersedes "logical range".

[1] crrev.com/196553003: Let FrameSelection remember the last Range set by Range.addRange()
[2] crrev.com/2653523003: Make DOMSelection cache Range
[3] https://www.w3.org/TR/selection-api/

BUG= 693557 
TEST=webkit_unit_tests --gtest_filter=FrameSelectionTest.FirstRange

Review-Url: https://codereview.chromium.org/2723133004
Cr-Commit-Position: refs/heads/master@{#454831}

[modify] https://crrev.com/67adef739b13e794850c228f3677dd75614958c1/third_party/WebKit/Source/core/editing/FrameSelection.cpp
[modify] https://crrev.com/67adef739b13e794850c228f3677dd75614958c1/third_party/WebKit/Source/core/editing/FrameSelectionTest.cpp
[modify] https://crrev.com/67adef739b13e794850c228f3677dd75614958c1/third_party/WebKit/Source/core/editing/SelectionEditor.cpp
[modify] https://crrev.com/67adef739b13e794850c228f3677dd75614958c1/third_party/WebKit/Source/core/editing/SelectionEditor.h