Fix -webkit-column-span isValueID ASSERT

In r356767 I changed consumeInteger to generate CSSPrimitiveValue's
of type Integer, however CSSPrimitveValueMappings.h still assumes
Number, so change that assumption to Integer.

BUG=549560

Review URL: https://codereview.chromium.org/1409063004

Cr-Commit-Position: refs/heads/master@{#358221}
3 files changed