Add a simpler key type for scratch resource keys.

BUG=skia:2889

Review URL: https://codereview.chromium.org/815833004
Fix computation of bound in FillBounds::updateSaveBounds

intersect doesn't change the bounds when the two bounds do not intersect. This is definitely not the intended behavior.

With the SKPs captured on 12/23/14, Chrome began passing Skia drawPicture ops that did not intersect the current clip - which revealed this bug.

Review URL: https://codereview.chromium.org/817483004