Track if all stages modulate inputColor in computeInvariantOutput

BUG=skia:

Review URL: https://codereview.chromium.org/647863002
Add GrLayerCache::writeLayersToDisk

I'm tired of readding this capability whenever I need to debug.

Review URL: https://codereview.chromium.org/654653006
Adjust skimage expectations for Android reference device

BUG=skia:3012

Review URL: https://codereview.chromium.org/650223004