[css-typed-om] Update paint style map behaviour for invalid properties.

Custom paint callbacks receive a computed style map (implemented as
FilteredComputedStylePropertyMap) that contain only properties specified
in inputProperties(). For forwards-compatiblity, inputProperties() is
allowed to have "invalid" or shorthand properties, which are not allowed
to be in the styleMap.

As resolved in [1], the style map should ignore any invalid/shorthand
properties. Furthermore, if a custom property is specified in
inputProperties(), it should have a default value of CSSUnparsedValue()
(not "null").

These changes also require changes in custom paint tests. Namely:

- Can no longer get "border-radius" from the style map. Have to use
  a longhand property like "border-left-radius".

- Getting custom properties like "--bar" will have a default value of
  CSSUnparsedValue.

[1] https://github.com/w3c/css-houdini-drafts/issues/523

Bug: 803690
Change-Id: I148b52568de698f69ee0bf594a2b92a9751ea484
Reviewed-on: https://chromium-review.googlesource.com/874984
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530763}
7 files changed