Bug 856932 - Fix fixed margins on position:fixed hierarchies. r=roc

Make sure that fixed margins aren't applied to auto-positioned frames, unless
their containing block is the primary frame of the body element.
Bug 856932 - Fix async scrolling of auto-positioned fixed position elements. r=nrc

Layers representing auto-positioned fixed position elements should not be
offset by the fixed layer margins.
Bug 860104 - Disable another click test, a=test-only
Backed out changeset c2449e6144d1 (wrong bug #)
Bug 838607 - Disable another click test, a=test-only