[DevTools] Improve emulation experience when responsive design mode is unavailable.

When emulation is unavailable (on device), only show the message about it.
When emulation is available, but without responsive design mode, move
toggle button to the OverridesView.

BUG=none

Review URL: https://codereview.chromium.org/358973003

git-svn-id: svn://svn.chromium.org/blink/trunk@177127 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove position: sticky

We would eventually like to implement position: sticky, but the current
implementation isn't designed in a way that integrates well with the existing
scrolling and compositing system. For example, position: sticky relies upon
updateLayerPositionsAfterDocumentScroll to function correctly, but that
function has no other purpose and can otherwise be removed. Similarly,
position: sticky doesn't work at all with composited overflow scrolling, which
is now the default mechanism for driving scrolling in the engine.

Once we've got our scrolling and compositing house in order, we should return
to position: sticky and implement the feature in a way that integrates well
with the rest of the engine. For now, however, this CL removes our current
implementation so we can focus on improving our implementation of the scrolling
features we've already shipped.

R=eseidel@chromium.org, esprehn@chromium.org
BUG=389638, 231752

Review URL: https://codereview.chromium.org/346603007

git-svn-id: svn://svn.chromium.org/blink/trunk@177128 bbb929c8-8fbe-4397-9dbb-9b2b20218538