print preview: more voice feedback issues.

print preview: more voice feedback issues.

* Removes an image without [alt] from being spoken
* Adds a label to summary (e.g. Printing: 1 sheet of paper.)
* Removes "Loading preview..." from screen readers (hidden with opacity: 0 only)
* Adds more info to the custom pages input ("e.g. 1-3, 5, 8-11")

R=dmazzoni@chromium.org
BUG=383955
Committed: https://crrev.com/1e5bccdca7ead7c5166d22613ff738acfd001037
Cr-Commit-Position: refs/heads/master@{#302226}

[dmazzoni, 2014-10-31 04:20:11]

The CQ bit was checked by dmazzoni@chromium.org

[dmazzoni, 2014-10-31 04:20:11]

lgtm

[commit-bot: I haz the power, 2014-10-31 04:20:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/697463002/1

[commit-bot: I haz the power, 2014-10-31 05:01:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-31 05:02:34]

Patchset 1 (id:??) landed as
https://crrev.com/1e5bccdca7ead7c5166d22613ff738acfd001037
Cr-Commit-Position: refs/heads/master@{#302226}