Bug 856264 - Part 1 - Failing tests for context menu. r=jimm

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extra : rebase_source : 06a8e9b6f39768e175224cf1e40889f10d979748
Bug 856264 - Part 2 - Ensure that menus get reshown properly across input methods. r=jimm

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extra : rebase_source : 45d8febeff971a8dd8454a242888267068d03878
Bug 895873 - Re-enable browser_context_menu_tests.js. Intermittent oranges likely fixed in bug 856264. r=jimm

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extra : rebase_source : c2e731fd5b061eda420e60cf959c39346df65003