This is just one of many canvas2D test regressions, even just one of many among the canvas compositing tests. Still, it may be useful to focus on one test for now. Test URL: http://philip.html5.org/tests/canvas/suite/tests/2d.composite.transparent.source-in.html Succeeds everywhere including B2G in default configuration Fails on B2G with skia/GL (as enabled by attachment 782802 [details] [diff] [review]).
Oh, also verified that this succeeds on B2G with specifically software Skia (not sure if that is the default). So the difference triggering the regression is specifically Skia/GL as opposed to software Skia.
In my local testing, this, like all other emulator reftest failures, is fixed by the emulator patch in bug 905141.