Re-enable pixel tests on the build bots Requested by rhogan on #webkit.
Since Blink forked none of the build bots are running pixel tests. This means that we are not catching painting regressions where the rendertree text dump is unaffected by the regression. Given that in the vast majority of cases a rebaseline to the pixel result also entails a rebaseline to the rendertree result and vice versa the additional overhead of enabling pixel tests and keeping them up to date is not the 'current effort x 2'. In most cases the rebaselining needs to be done anyway, the rebaseliner just needs to update the png result while they're at it.