Bug 261037. Support scrolled fieldsets. r=mats

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extra : rebase_source : 248521189921b2a46f0805582a3789b27c26ecbd
Bug 929021. Skip calling GetFrameBoundsForTransform when we don't need it. r=heycam

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extra : rebase_source : 646bea24724034ca4e2c0f2075ae8cb86f884023