Timeline: scale impl-side frames overview according to device pixel ratio

We used to have impl-side frames window height hardcoded and not scaled to
device pixel ratio, hence the window looked too small on hi-dpi devices.

TBR=pfeldman@chromium.org

BUG=

Review URL: https://codereview.chromium.org/113533006

git-svn-id: svn://svn.chromium.org/blink/trunk@164286 bbb929c8-8fbe-4397-9dbb-9b2b20218538