Put CSSURIValue into the blink::cssvalue namespace.

This is part of the gradual process of putting all types that derive
from CSSValue into blink::cssvalue:: namespace.

Bug: 667961
Change-Id: I6866ec8aea781b925167eb7632e2f66e296e6cde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524751
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Cr-Commit-Position: refs/heads/master@{#641513}