[Editing][Codehealth] Replace Position::uptream/downstream to mostBackward/ForwardCaretPosition.

[Editing][Codehealth] Replace Position::uptream/downstream to 
mostBackward/ForwardCaretPosition

Position::upstream calls just VisibleUnit.cpp::mostForwardCaretPosition and also 
downstream:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/c
ore/editing/Position.cpp&l=484&cl=GROK

This CL replaces it.
We will replace all upstream/downstream and remove them.

BUG=518738
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200977

[yoichio, 2015-08-21 05:00:13]

yoichio@chromium.org changed reviewers:
+ yosin@chromium.org

[yosin_UTC9, 2015-08-21 07:18:58]

yosin@chromium.org changed reviewers:
+ tkent@chromium.org

[yosin_UTC9, 2015-08-21 07:18:59]

lgtm

+tkent@ for OWNERS.

[tkent, 2015-08-21 07:58:14]

lgtm

[yosin_UTC9, 2015-08-21 08:01:22]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-21 08:01:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1306743002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1306743002/1

[commit-bot: I haz the power, 2015-08-21 09:31:24]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200977

[yoichio, 2015-08-21 10:10:54]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1304133002/ by yoichio@chromium.org.

The reason for reverting is: This patch misused each other.
See:
 https://codereview.chromium.org/1310433004/.