Add ability to sort clusters by commit timestamp.

Next CL will turn the sorting into a full element that allows sorting both
ascending and descending.

BUG=skia:

Review URL: https://codereview.chromium.org/728533003
Fixed Alpha Handling in Diff

It restores the previous behavior, but shows different colors
for pixels where the RGB differs and where the alpha differs (all pixels are assumed to be pre-multiplied).

That way the colors of the output pixel parallel what the user sees.

BUG=skia:2930

Review URL: https://codereview.chromium.org/719323002