Bug 795657: Integrate native viewport configuration better into async pan-zoom code. r=jwir3,roc a=blocking-basecamp This is a rollup of the following patches - Change the interpretation of FrameMetrics.mZoom to a "resolution-indepedent zoom", instead of a resolution-depedent scale factor. r=roc - Remove mention of "meta" from TabChild. r=roc - Remove some useless logging. r=roc - Tag FrameMetrics with its composition bounds at paint time. r=roc - Add a helper to calculate the render resolution for a FrameMetrics. r=roc - Add a helper to compute the approximate CSS dimensions a FrameMetrics will cover during composition. r=roc - BrowserElementScrolling doesn't actually care about zoom or resolution. r=roc - Accept the viewport that content has calculated, when it's received the latest widget geometry update. r=roc - Mechanically separate uses of zoom/resolution based on new definitions. r=roc - Convert GetViewportInfo()'s resolution-dependent scale into resolution-indepedent zoom. r=roc - Reinterpret defaultZoom == 0.0 as "intrinsic scale". r=jwir3,roc

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