DevTools: properly clear layers when doing a partial replay of SkPicture

We used to clear only current (top) layer, so one could occasionaly see
things painted before the start of replay window as the saved layer was
popped. Now we just clear an old layer before it's saved if it happens
before replay window start.

Drive-by: remove one case where logging canvas could potentially count
operations differently than other canvases.

Review URL: https://codereview.chromium.org/418173009

git-svn-id: svn://svn.chromium.org/blink/trunk@179417 bbb929c8-8fbe-4397-9dbb-9b2b20218538