[css-properties-values-api] Substitution behavior.

[chromium-wpt-export-bot]

Currently, registered properties substitute into other values exactly
like specified (like unregistered properties). This means that, for
a <length>-registered property with a specified value "10em" (for
instance), when that property is substituted via var()-reference,
the tokens "10em" are inserted. This is not correct, and produces the
wrong result with e.g. inherited values.

This CL changes that, by implementing "absolutization" of registered
custom properties: a process which calculates the computed value of
the property, and then produces a token stream usable for substitution
which is equivalent to the computed value.

• Currently we resolve var()-references on all custom properties
  before applying high-priority properties. This is no longer
  possible, because the true value of a custom property (also
  unregistered) can not be known until the font has been updated.
  Consider: --reg-len: 1em; --unreg:var(--reg-len). Here, the
  computed value of --unreg should be "16px" (assuming a font-size
  of 16px), and not "1em". If we destructively resolve --unreg
  before the font size is known, we end up with the wrong tokens.

  Hence the resolution process has been moved to after the font
  has been updated.

• Custom properties must also be usable from high-priority properties.
  Since they are no longer resolved beforehand, they are resolved
  "on the fly", non-destructively. "Non-destructively" means that
  resolved token streams are not stored on ComputedStyle, such that
  any var()-references are kept for the real resolution pass after
  the font has been updated.

  This works, because the high-priority properties do not require
  proper "absolute substitution" to produce the correct value.

  Note that font-size is special, in that font-relative units may
  not be used if they arrive via a registered custom property.

• There is a special resolving pass for registered custom properties
  (ComputeRegisteredVariables). This pass produces non-absolute CSSValues
  for calculation of animation.

• Parsing of registered properties now happens entirely in
  CSSVariableResolver, and no longer in variable.cc. Having this in
  multiple places is just confusing.

R=futhark@chromium.org

Bug: 641877
Change-Id: Ic705d0808ffcea0ae5db02fb20870767175bb706
Reviewed-on: https://chromium-review.googlesource.com/1240274
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593902}

[wpt-pr-bot]

Already reviewed downstream.