Bug 1167882 - Add utility functions MaxScaleRatio() and MinScaleRatio() to Units.h. Bug 1167882 - Use the display size (includes scrollbar areas) rather than the root composition bounds (excludes scrollbar areas) to calculate the intrinsic scale.
Fix a potential null-deref in TabChild::HandlePossibleViewportChange. Bug 1164406 - Exclude the scrollbars from the composition bounds in TabChild::HandleCalculateViewportChange() when appropriate.
Bug 1125325 - Make TabParent/TabChild UpdateDimensions messages aware of the display scale.