Introduce Position::IsEquivalent()

Introduce Position::IsEquivalent()

This patch introduces |Position::IsEquivalent()| to check whether two positions
are in same position regardless anchor type, e.g. |Position(node, 0)| and
|Position::FirstPositionInNode(node)|. denote a position before first child of
|node|.

This patch is a preparation of patch[1].

[1] http://crrev.com/2875933004: Utilize Position::IsEquivalent() in
{Up,Down}streamIgnoringEditingBoundaries()

BUG=716093, 719343
TEST=run_webkit_unit_tests --gtest_filter=PositionTest.IsEquivalent

Review-Url: https://codereview.chromium.org/2875253002
Cr-Original-Commit-Position: refs/heads/master@{#471689}
Review-Url: https://codereview.chromium.org/2890403002 .
Cr-Commit-Position: refs/branch-heads/3071@{#628}
Cr-Branched-From: a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed: https://chromium.googlesource.com/chromium/src/+/b67267c9b096b7ac8fd187bccd91178c04689e32

[yosin_UTC9, 2017-05-19 10:30:00]

Description was changed from

==========
Introduce Position::IsEquivalent()

This patch introduces |Position::IsEquivalent()| to check whether two positions
are in same position regardless anchor type, e.g. |Position(node, 0)| and
|Position::FirstPositionInNode(node)|. denote a position before first child of
|node|.

This patch is a preparation of patch[1].

[1] http://crrev.com/2875933004: Utilize Position::IsEquivalent() in
{Up,Down}streamIgnoringEditingBoundaries()

BUG=716093, 719343
TEST=run_webkit_unit_tests --gtest_filter=PositionTest.IsEquivalent

Review-Url: https://codereview.chromium.org/2875253002
Cr-Commit-Position: refs/heads/master@{#471689}
(cherry picked from commit 4aee85c54527cbd0857b07d6dc4699465f55a18f)
==========

to

==========
Introduce Position::IsEquivalent()

This patch introduces |Position::IsEquivalent()| to check whether two positions
are in same position regardless anchor type, e.g. |Position(node, 0)| and
|Position::FirstPositionInNode(node)|. denote a position before first child of
|node|.

This patch is a preparation of patch[1].

[1] http://crrev.com/2875933004: Utilize Position::IsEquivalent() in
{Up,Down}streamIgnoringEditingBoundaries()

BUG=716093, 719343
TEST=run_webkit_unit_tests --gtest_filter=PositionTest.IsEquivalent

Review-Url: https://codereview.chromium.org/2875253002
Cr-Original-Commit-Position: refs/heads/master@{#471689}
Review-Url: https://codereview.chromium.org/2890403002 .
Cr-Commit-Position: refs/branch-heads/3071@{#628}
Cr-Branched-From:
a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed:
https://chromium.googlesource.com/chromium/src/+/b67267c9b096b7ac8fd187bccd91...
==========

[yosin_UTC9, 2017-05-19 10:30:03]

Committed patchset #1 (id:1) manually as
b67267c9b096b7ac8fd187bccd91178c04689e32.