Rollup of bug 819000: Preload about:blank. r=cjones,jlebar a=cjones

Includes the following patches

Bug 819000: Bail out of PresShell::Paint if not active. r=roc
Bug 819000: Make sure preloaded docshells are reset to active. r=jlebar
Bug 819000: Preload about:blank. r=cjones a=cjones