DevTools: draw realistic chrome on top of viewport in layers3dview

DevTools: draw realistic chrome on top of viewport in layers3dview

- add graphics for chrome on top of viewport rectange;
- make viewport border light-grey, let its width scale along with
    contents scale.
- change Viewport checkbox to Chrome (always show viewport)
- (drive by) properly handle potential image/texture creation errors
- (drive by) disable user selection on Layers3DView in CSS as annoying.

BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183906

[caseq, 2014-10-16 12:57:14]

caseq@chromium.org changed reviewers:
+ pfeldman@chromium.org

[caseq, 2014-10-16 13:02:35]

Screenshot:
https://drive.google.com/a/chromium.org/file/d/0B_yi9xtujhXSb3g4c2xkY0ZyR3c/v...

[pfeldman, 2014-10-17 14:57:59]

lgtm

[caseq, 2014-10-17 15:51:31]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-10-17 15:51:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/661753002/20001

[commit-bot: I haz the power, 2014-10-17 17:21:39]

Committed patchset #2 (id:20001) as 183906