DevTools: [CodeMirror] fix rendering of errors in editor on retina

The patch fixes inappropriate background image sizes for a red wave.

The patch also corrects horizontal wave position by taking a padding between text line and gutter into account.

BUG=417683
R=apavlov, pfeldman

Review URL: https://codereview.chromium.org/608483002

git-svn-id: svn://svn.chromium.org/blink/trunk@182683 bbb929c8-8fbe-4397-9dbb-9b2b20218538