For the purpose of rendering, Blink must calculate all pending slot assignments
before UpdateStyle phase. Unless that, a recursive RecalcStyle() may not
traverse a node which should be re-attached.

e.g. Suppose the following tree, where #d1 is assigned to a slot.

host
├──/shadow-root
│   └── slot name=s1
└── div id=d1 slot=s1

Then, we change #d1's slot attribute to s2, like:

> document.querySelector('#d1').setAttribute('slot', 's2');

host
├──/shadow-root
│   └── slot name=s1
└── div id=d1 slot=s2

In this case, #d1 should be removed from a LayoutTree. In other words, Blink
has to reattach #d1 somehow. However, if we don't recalc a slot assignment for
the shadow tree before UpdateStyle, a recursive RecalcStyle never traverses the
sub-tree because child-needs-style-recalc flag is not set for the node (and its
ancestor nodes). A flag should be set as a result of slot assignment recalc.

Thanks to the Incremental Shadow DOM, a slot assignemt recalc now becomes a
local operatoin on each shadow tree, rather than one global operation for every
shadow trees. We don't need to traverse a composed tree to recalc all. We can
recalc a slot assignment for each shadow tree directly, without traversing a
composed tree.

For a shadow tree which is not connected, we don't need to recalc its slot
assignment before UpdateStyle because such a shadow tree shouldn't have any
effect on rendering. Lazy slot assignment recalc is enough for such a shadow
tree.

This CL doesn't introduce any optimization to minimize the number of
to-be-reattached nodes. I'll optimize that as a next task. I'll use a sort of
dynamic programming there, as I did at
https://chromium-review.googlesource.com/c/chromium/src/+/535493 for
non-Incremental Shadow DOM.

Except for the performance, this CL should be the last part of Incremental
Shadow DOM from the external behavior's perspective. Style and Layout should
work correctly after this CL, even with Incremental Shadow DOM.

BUG=776656

Change-Id: Id18e87ff59d92863c68c571e7db09253c08aa91f
Reviewed-on: https://chromium-review.googlesource.com/964062
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546458}