Split out String, URI and CustomIdent from CSSPrimitiveValue

Remove String, URI and CustomIdent from CSSPrimitiveValue and moved them
into 3 new classes: CSSStringValue, CSSURIValue and CSSCustomIdentValue.

BUG=523893

Review URL: https://codereview.chromium.org/1306823004

Cr-Commit-Position: refs/heads/master@{#352795}
31 files changed