[css-typed-om] Refactor CSSValue coercion.

In CoerceStyleValuesOrStrings, instead of temporarily storing CSSValues
in a vector, then moving those values to a CSSValueList, store a
CSSStyleValueVector, and perform the CSSStyleValue -> CSSValue conversion
during the CSSValueList construction.

This is a preparatory CL for a subsequent change where it's necessary to
have a CSSStyleValueVector of the incoming values.

R=chrishtr@chromium.org

Bug: 641877
Change-Id: I76bd76c42c4194772b72f80c3ca86d57848a1c72
Reviewed-on: https://chromium-review.googlesource.com/c/1270997
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598634}
3 files changed