Handle AX help-Text for media time slider.

Handle AX help-Text for media time slider.

The AX help-text for media time slider is used for audio and video
with "movie time scrubber". But it's not proper to audio time slider.
This patch adds help-text for audio time slider, makes 
help-text check if it is audio or video and applies proper help-text.

BUG=431233
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=185539

[je_julie(Not used), 2014-11-17 23:44:44]

je_julie.kim@samsung.com changed reviewers:
+ dmazzoni@chromium.org, mkwst@chromium.org

[je_julie(Not used), 2014-11-18 00:03:59]

Dominic and Mike,

This patch handles help-text for audio time slider on Blink
and also moves the API to check if it's video control to 'AXNodeObject' because
it should be used for 'AccessibilityMediaTimeline' as well and it's not from
'AccessibilityMediaControl'.

I'll handle this patch first and then I'll request a review for the patch adding
string resources.
FYI, the patch adding string resource on chromium is 
https://codereview.chromium.org/728223004/.

PTAL.

[Mike West, 2014-11-18 07:54:05]

LGTM.

[dmazzoni, 2014-11-18 16:57:03]

The CQ bit was checked by dmazzoni@chromium.org

[dmazzoni, 2014-11-18 16:57:03]

lgtm

[commit-bot: I haz the power, 2014-11-18 16:58:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/729343003/1

[commit-bot: I haz the power, 2014-11-18 18:19:20]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=185539