Isolation failures in gecko reftests

[Gankra]

Accidentally noticed that the raw logs of recent gecko try builds include these errors (on a passing test).

Try build: https://treeherder.mozilla.org/#/jobs?repo=try&revision=79738b756f0e08958e493f3020b254817a97b94a&selectedJob=115176289

Relevant raw log output:

[task 2017-07-18T10:31:45.277463Z] 10:31:45     INFO - REFTEST TEST-START | file:///home/worker/workspace/build/tests/reftest/tests/layout/reftests/async-scrolling/background-blend-mode-1.html == file:///home/worker/workspace/build/tests/reftest/tests/layout/reftests/async-scrolling/background-blend-mode-1-ref.html
[task 2017-07-18T10:31:45.278803Z] 10:31:45     INFO - REFTEST INFO | RESTORE PREFERENCE pref(apz.disable_for_scroll_linked_effects,false)
[task 2017-07-18T10:31:45.280233Z] 10:31:45     INFO - REFTEST TEST-LOAD | file:///home/worker/workspace/build/tests/reftest/tests/layout/reftests/async-scrolling/background-blend-mode-1.html | 1461 / 1945 (75%)
[task 2017-07-18T10:31:45.346178Z] 10:31:45     INFO - ERROR:webrender::frame_builder: Isolation conflict detected on StackingContextIndex(6): Full
[task 2017-07-18T10:31:45.350652Z] 10:31:45     INFO - ERROR:webrender::frame_builder: Isolation conflict detected on StackingContextIndex(6): Full
[task 2017-07-18T10:31:46.318003Z] 10:31:46     INFO - ERROR:webrender::frame_builder: Isolation conflict detected on StackingContextIndex(6): Full
[task 2017-07-18T10:31:46.318122Z] 10:31:46     INFO - REFTEST TEST-LOAD | file:///home/worker/workspace/build/tests/reftest/tests/layout/reftests/async-scrolling/background-blend-mode-1-ref.html | 1461 / 1945 (75%)
[task 2017-07-18T10:31:46.436053Z] 10:31:46     INFO - ERROR:webrender::frame_builder: Isolation conflict detected on StackingContextIndex(6): Full
[task 2017-07-18T10:31:46.436187Z] 10:31:46     INFO - ERROR:webrender::frame_builder: Isolation conflict detected on StackingContextIndex(6): Full
[task 2017-07-18T10:31:47.847113Z] 10:31:47     INFO - ERROR:webrender::frame_builder: Isolation conflict detected on StackingContextIndex(6): Full
[task 2017-07-18T10:31:47.848267Z] 10:31:47     INFO - REFTEST TEST-PASS | file:///home/worker/workspace/build/tests/reftest/tests/layout/reftests/async-scrolling/background-blend-mode-1.html == file:///home/worker/workspace/build/tests/reftest/tests/layout/reftests/async-scrolling/background-blend-mode-1-ref.html | image comparison
[task 2017-07-18T10:31:47.849045Z] 10:31:47     INFO - REFTEST TEST-END | file:///home/worker/workspace/build/tests/reftest/tests/layout/reftests/async-scrolling/background-blend-mode-1.html == file:///home/worker/workspace/build/tests/reftest/tests/layout/reftests/async-scrolling/background-blend-mode-1-ref.html

This shows up fairly consistently. But it looks like it's only this one reftest (at least, in this task).

[glennw]

Fixed by #2031. We can open new bugs for any isolation specific issues.