Fix Volume slider is captured in screenshot done in touchview mode
Solution:
1. User holds volume-down key.
2. Remember the non-repeated pressed volume.
3. Immediately start reducing volume.
4. If the power key is tapped,
4a TrayAudio hides the volume slider.
4b Taking the screenshot.
4c Restore the volume (do not notify UI)
BUG=628479
TEST=tested on veyron_minnie touchview mode and it works fine. The only defect I can image is taking screenshot when the device is playing audio. If volume-down is presssed for a long time, you can feel audio volume is firstly decreased and then restored back.
Review-Url: https://codereview.chromium.org/2190773002
Cr-Commit-Position: refs/heads/master@{#410145}
10 files changed
