Switch the remaining classes in core/css to use downcast helpers (7/n)

The following classes are migrated:

- CSSLayoutFunctionValue
- CSSPaintValue
- CSSInheritedValue
- CSSInitialValue
- CSSPathValue
- CSSRayValue
- CSSQuadValue
- CSSReflectValue
- CSSShadowValue
- CSSStringValue
- CSSCubicBezierTimingFunctionValue
- CSSStepsTimingFunctionValue
- CSSFramesTimingFunctionValue
- CSSUnicodeRangeValue
- CSSURIValue
- CSSVariableReferenceValue
- CSSInvalidVariableValue
- CSSPendingSubstitutionValue
- CSSImageGeneratorValue
- CSSInitialValue

R=jbroman@chromium.org
CC=blink-reviews-vendor@chromium.org

BUG=891908

fix

Change-Id: I9421fc824715e34ef5aed038683fa16fde5da056
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1516452
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639730}
49 files changed