The first failing build is after Webkit r83556, so it may be related to bug 58308.
Reproducible upon reloading the page, with the same message printed to the console as in bug 58306. Assigning; close this one as a duplicate when the other is confirmed resolved.
FYI: the only line in test_expectations with this bug ID is
BUGWK58328 GPU WIN LINUX DEBUG : compositing/webgl/webgl-reflection.html = IMAGE
(I.E., not Mac as this bug suggests). It also doesn't seem to have failed on any platform since it was rebaselined in r90887.
I'm removing from test_expectations; I'll leave it up to John to decide if this bug should be closed.
I can confirm that this is no longer reproducible in a top of tree Chromium debug build.
LayoutTest failures for Chromium are being marked WontFix. The Bug is still accessible and referenced from TestExpectations.