DevTools: draw load/first paint marks as tall vertical delimiters

BUG=424500

Review URL: https://codereview.chromium.org/706313002

git-svn-id: svn://svn.chromium.org/blink/trunk@184990 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Migrate elements panel undo/redo shortcuts to declarative actions.

R=apavlov

Review URL: https://codereview.chromium.org/688513004

git-svn-id: svn://svn.chromium.org/blink/trunk@184991 bbb929c8-8fbe-4397-9dbb-9b2b20218538