Bug 967844. Part 1: Move mBackgroundColor from Layer to FrameMetrics. r=kats

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extra : rebase_source : b5fd1fd1b0b1f55990e72d205cc871a46d5f4f8c
Bug 967844. Part 2: Move mContentDescription from Layer to FrameMetrics. r=kats

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extra : rebase_source : d5a2d8ea13369db215e96e721efc3ce0b354a523
Bug 967844. Part 3: Setup FrameMetrics from FrameLayerBuilder based on animated geometry roots. r=mattwoodrow

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extra : rebase_source : d30d2af626038cc556fcd3add9a3876be2c58239