cc: Fix draw transforms for descendants of fixed-position layers

cc: Fix draw transforms for descendants of fixed-position layers

A fixed-position layer's transform tree parent may be an ancestor
of the layer's render target's transform tree node, since layers
on the path from the fixed-position layer to its containing layer
may own render surfaces. DrawTransformFromPropertyTreesInternal
currently works around this by accounting for the transform
between the render target and the transform tree parent's render
target when computing the draw transform of fixed-position layers.
However, that workaround doesn't handle the descendants of
fixed-position layers.

This CL removes that workaround altogether, and instead always
creates nodes for fixed-position layers. The target_id for
these nodes is the id of the fixed-position layer's actual
render target's transform node. Since (as explained above) this
node may not be an ancestor of the fixed-position layer's
transform node, this CL also weakens the restriction on the
inputs to TransformTree::ComputeTransform.

BUG=484223

Review URL: https://codereview.chromium.org/1128453002

Cr-Commit-Position: refs/heads/master@{#328459}