[css-properties-values-api] Proper types from StylePropertyMap iterator.

When iterating a StylePropertyMap, any registered custom properties in
that map must have the appropriate type according to the registration.
Currently all custom properties are always CSSUnparsedValue, which is only
correct for unregistered properties and properties registered as "*".

Fixed by parsing the value on its way out, similar to how get/getAll
implements the same behavior.

R=chrishtr@chromium.org

Bug: 641877
Change-Id: I5e0b2d064cd0ce4827eabf464a5cf0a0e138c5db
Reviewed-on: https://chromium-review.googlesource.com/c/1264540
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598247}
3 files changed