Files.app: Stop to use autogen IDs of TreeItems to identify tree items.

Files.app: Stop to use autogen IDs of TreeItems to identify tree items.

The IDs are generated by counter, so sometime different IDs were assigned to the
items and it causes flakiness of browser tests.

BUG=512669
TEST=None

Committed: https://crrev.com/e95aca9224bb05f69ec9e695b52b49ca143672a1
Cr-Commit-Position: refs/heads/master@{#339848}

[hirono, 2015-07-22 07:49:48]

hirono@chromium.org changed reviewers:
+ fukino@chromium.org

[hirono, 2015-07-22 07:49:49]

PTAL, thanks!

[fukino, 2015-07-22 07:55:02]

LGTM! Thanks!

[hirono, 2015-07-22 07:55:14]

The CQ bit was checked by hirono@chromium.org

[hirono, 2015-07-22 07:55:21]

On 2015/07/22 07:55:02, fukino wrote:
> LGTM! Thanks!

Thanks!

[commit-bot: I haz the power, 2015-07-22 07:55:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1246923003/20001

[commit-bot: I haz the power, 2015-07-22 08:38:22]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-07-22 08:39:08]

Patchset 2 (id:??) landed as
https://crrev.com/e95aca9224bb05f69ec9e695b52b49ca143672a1
Cr-Commit-Position: refs/heads/master@{#339848}