Bug 1064886: add "Enable E10S" checkbox to preferences, r=felipe

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extra : rebase_source : 34139fb9f2074b73b4cb5e6dcbad7357fff9149d
Bug 1064885: add prompt to encourage Nightly users to enable e10s, r=felipe

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extra : rebase_source : cf288b2d3b80c540bcc0ba4d2e2caae8b7a95925