DevTools: Use dithering to draw timeline events overview

Make timeline event overview depict real events density.

BUG=437199

Review URL: https://codereview.chromium.org/758003003

git-svn-id: svn://svn.chromium.org/blink/trunk@186325 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: increase the min sidebar width to make sure close button is always visible.

BUG=438116

Review URL: https://codereview.chromium.org/761783003

git-svn-id: svn://svn.chromium.org/blink/trunk@186327 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Support worker cpu profiles save/load on timeline.

BUG=363976

Review URL: https://codereview.chromium.org/770823002

git-svn-id: svn://svn.chromium.org/blink/trunk@186331 bbb929c8-8fbe-4397-9dbb-9b2b20218538