Merge odd items and even items of FormDataList::m_items.

Merge odd items and even items of FormDataList::m_items.

FormDataList::Item class represents a name string in a form data, or a value in
a form data.  A value can be a string or Blob, but a name string is always a
string.  Representing a name string by Item class is overkill.  This CL adds
m_key member to Item class.

Odd items in FormDataList::m_items represented name strings, and even items in
FormDataList::m_items represented values. Now each of items in
FormDataList::m_items represents a pair of a name string and a value.  This
makes the code much simpler.

This CL has no behavior changes.

BUG=528840
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201862

[tkent, 2015-09-07 07:24:50]

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

[tkent, 2015-09-07 07:24:51]

Keishi, would you review this please?

[keishi, 2015-09-07 07:28:47]

LGTM

[tkent, 2015-09-07 07:30:18]

The CQ bit was checked by tkent@chromium.org

[commit-bot: I haz the power, 2015-09-07 07:30:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1314013007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1314013007/1

[commit-bot: I haz the power, 2015-09-07 07:36:10]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201862