Chromevox2: make sure the braille cursor is shown for empty text fields.

Chromevox2: make sure the braille cursor is shown for empty text fields.

The empty string in a boolean context led to the cursor not being
shown.  Changed to compare against undefined instead.
This also fixes the value start offset to always be 0. It should only be
non-zero for multiline text fields where only part of the field is included in
the braille output.

BUG=

Committed: https://crrev.com/60383b1f0112b73aa4ab0d52ab053924c7927481
Cr-Commit-Position: refs/heads/master@{#321543}

[Peter Lundblad, 2015-03-19 15:17:22]

plundblad@chromium.org changed reviewers:
+ dtseng@chromium.org

[David Tseng, 2015-03-19 17:13:32]

lgtm

[Peter Lundblad, 2015-03-20 10:02:44]

The CQ bit was checked by plundblad@chromium.org

[commit-bot: I haz the power, 2015-03-20 10:03:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1019283003/1

[commit-bot: I haz the power, 2015-03-20 10:31:54]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-20 10:32:56]

Patchset 1 (id:??) landed as
https://crrev.com/60383b1f0112b73aa4ab0d52ab053924c7927481
Cr-Commit-Position: refs/heads/master@{#321543}