Bug 837113: Close button in tabs ui. [r=mfinkle]

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extra : rebase_source : 52af4d8e0316be67838fcd5a463a5fdcbac124c2
Bug 831021: Tab button shouldn't overlap with browser-toolbar. [r=mfinkle]

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extra : rebase_source : 998f9e9ad22fbe010a2fee1cab83fd5ef93c91c3