Bug 891035: Provide mutual exclusion in Plugin Hang UI to protect against concurrent shutdown and user response. r=bsmedberg

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extra : rebase_source : c7960c2d74af8af57aa7e97867df61a975f4d7fe
Bug 891388. Implement Arc() in PathCG. r=joe

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extra : rebase_source : 3ebc541ce7407eba02165366bcc754484f65fcaa