There are 2 failures here in the last 7 days, on try.
Bug 1640835 Comment 5 Edit History
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Retriggers: https://treeherder.mozilla.org/#/jobs?repo=autoland&searchStr=Android%2C7.0%2Cx86-64%2CWebRender%2Cdebug%2CMochitests%2Ctest-android-em-7.0-x86_64-qr%2Fdebug-geckoview-mochitest-plain-e10s%2C4&group_state=expanded&fromchange=db090162113b817aab48a425f64b12b96d7fda51&tochange=fe71d126d639dbf96e6b5b87a3cc33c0fb9d0676&selectedTaskRun=cRWVMGqpRuC-NZg5Musx1Q.0 Dan can you take a look if this is really from Bug 976669?