Drop a "using namespace WTF" in V8CSSStyleDeclarationCustom.cpp

global "using namespace WTF" causes compiler warnings in jumbo builds
and in general extend the global namespace a lot which increases the
risk of symbol conflicts.

This patch changes a "using namespace WTF" to an explicit
WTF::CopePointCompareLessThan.
Review-Url: https://codereview.chromium.org/2975603002
Cr-Commit-Position: refs/heads/master@{#484938}