Move flag isValid from SVGSynchronizableAnimatedProperty to SVGRect.

The flag was introduced in r146495 to conform to empty/invalid viewBox attribute spec, but not being used elsewhere. This patch moves the flag inside SVGRect to simplify SVGSynchronizableAnimatedProperty.

TEST=svg/custom/viewBox-empty.html
BUG=308818

Review URL: https://codereview.chromium.org/28063002

git-svn-id: svn://svn.chromium.org/blink/trunk@160515 bbb929c8-8fbe-4397-9dbb-9b2b20218538