Bug 827844 - Fix flickering when layout changes on Firefox/android. r=kats,bgirard

Fix flickering that can occur when the surface size changes due to a layout
change (such as the virtual keyboard appearing/disappearing) on Android.
Bug 828249 - Fix fixed layer positioning when zoomed out beyond page bounds. r=kats

width/height parameters in CompositorParent got mixed up at some point.
Bug 828345 - Maintain aspect ratio for clamping scrollport. r=kats

Make sure the clamping scroll-port maintains its aspect ratio on Android. This
fixes fixed position elements appearing to jitter after zooming out beyond the
page boundaries.