Bug 904926. Remove unnecessary lock. r=cpearce

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extra : rebase_source : 0088907b65ff3ce2e1f15b2bb1b5e0cdb7126f24
Bug 909157. Invalidate rendering observers when an image changes. r=mattwoodrow

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extra : rebase_source : 04b127fde9a9e07e50c6196f53a1eea3c8f04db6