We've seen frequent intermittent failures in WebGL tests since WebGL AA landed. This turns out to be OOM conditions leading to failure to create WebGL contexts, due to the fact that the JS engine keeps too many unreferenced WebGL contexts around before it decides to GC (bug 617453). This patch makes us trigger the GC before every WebGL test page (there are 300 of them) which has proved to solve the problem on tryserver, with 18 green runs without any orange: https://tbpl.mozilla.org/?tree=Try&rev=789cf49fdc63
Failures occured most of the time on 32bit linux-pgo but have been observed on 64bit too, see bug 695947 comment 6.
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