Revert of [DevTools] Device mode: reset page scale when enabling and improve UI. (https://codereview.chromium.org/389573002/)

Reason for revert:
Speculative revert. A long list of compositing related layout tests began failing on the Mac Retina bot.  Some of the tests look like they could just be rebaselined, others are true regressions and seem related to page scale. Regression range is 178044:178046 This is the only change in that range that is even remotely related to graphics.

Original issue's description:
> [DevTools] Device mode: reset page scale when enabling and improve UI.
> 
> BUG=384563
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178045

TBR=pfeldman@chromium.org,dgozman@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=384563

Review URL: https://codereview.chromium.org/391753002

git-svn-id: svn://svn.chromium.org/blink/trunk@178058 bbb929c8-8fbe-4397-9dbb-9b2b20218538