[DevTools] Do not rescale inspected page on DevTools zoom.

When zooming, we change split size. This requires to ignore constraints
since total DevTools body size will be rescaled later by calling
InspectorFrontendHost.setZoomFactor.

BUG=318751

Review URL: https://codereview.chromium.org/108713007

git-svn-id: svn://svn.chromium.org/blink/trunk@164781 bbb929c8-8fbe-4397-9dbb-9b2b20218538