Simplify SVGAnimateElement::HasValidAnimation()

At this point we will have resolved the type and any CSS property that
'attributeName' maps to, so we can use the (stricter) test
IsAnimatingCSSProperty() instead of IsTargetAttributeCSSProperty().
Similarly we can drop the AnyQName() test because it shouldn't map to
a proper type/CSS property.

Remove IsTargetAttributeCSSProperty() since it is now unused.

Bug: 1017723
Change-Id: I12d231718b335e1c21cff98247fe4869fd3232c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537837
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#827362}