This is a rollup of the following patches
Bug 811315, part 0.9: Fix pre-existing race condition that allows dying ContentParents to accidentally try to load new TabParents. r=jlebar
Bug 811315, part 0.91: Fix another pre-existing race condition where FirstIdle might run after ContentChild::ActorDestroy(). r=jlebar
Bug 811315, part 1: Make "browser" process limit infinite and unload the processes when all tabs are closed. Firefox with process-per-tab! r=jlebar
Bug 811315, part 2: Pass scrolling-behavior hint down into TabParent instead of inferring it from app/browser. r=jlebar
Bug 811315, part 3: Add a mozasyncpanzoom attribute for iframes and honor it when constructing a remote frame. r=jlebar sr=roc