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

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extra : rebase_source : d7cf8756678cc342e1d648638dd867c5479b74ba
Bug 967844. Part 2: Move mContentDescription from Layer to FrameMetrics. r=kats

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extra : rebase_source : b616a0ab2e5bb203a66a5f0b3644bdcbe5f17ce7
Bug 967844. Part 3: Setup FrameMetrics from FrameLayerBuilder based on animated geometry roots. r=mattwoodrow

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extra : rebase_source : 19593cafc17053d450905161fa7d4ce8721c58a0