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Introduces the pref `webgl.testing.context_creation_error`, to force creation of a new WebGLRenderingContext to fail.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#17038 (github issue number if applicable).
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- [X] There are tests for these changes OR
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Source-Repo: https://github.com/servo/servo
Source-Revision: 3db7f5d556d85ff6af9bb81f693a2c65e6e791fa
--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 19078df81cf2bd36ab232a25d7c68976ffc7c673
1cc7429f72
