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=

Review URL: https://codereview.chromium.org/661753002

git-svn-id: svn://svn.chromium.org/blink/trunk@183906 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: [Workspace] fix removing stylesheet-related uiSourceCodes

The patch fixes a bug in removing uiSourceCodes from workspace as
the associated stylesheet gets removed.

BUG=424544
R=vsevik

Review URL: https://codereview.chromium.org/661923005

git-svn-id: svn://svn.chromium.org/blink/trunk@183915 bbb929c8-8fbe-4397-9dbb-9b2b20218538