Revert 159503 "URLUtils: Use ImplementedAs for toString"

Build failure on all platforms.

gen/blink/bindings/V8HTMLAnchorElement.cpp:673:34:error: 'toString' is not a member of 'WebCore::DOMURLUtils'
gen/blink/bindings/V8URL.cpp:390:34:error: 'toString' is not a member of 'WebCore::DOMURLUtils'
gen/blink/bindings/V8HTMLAreaElement.cpp:521:34:error: 'toString' is not a member of 'WebCore::DOMURLUtils'
gen/blink/bindings/V8WorkerLocation.cpp:170:34:error: 'toString' is not a member of 'WebCore::DOMURLUtilsReadOnly'

> URLUtils: Use ImplementedAs for toString
> 
> This is a second try. The first time the builders had some snafu
> 
> Original: https://src.chromium.org/viewvc/blink?revision=159476&view=revision
> Revert:   https://src.chromium.org/viewvc/blink?revision=159477&view=revision
> 
> BUG=None
> TBR=abarth@chromium.org
> 
> Review URL: https://codereview.chromium.org/27053003

TBR=arv@chromium.org

Review URL: https://codereview.chromium.org/27096004

git-svn-id: svn://svn.chromium.org/blink/trunk@159506 bbb929c8-8fbe-4397-9dbb-9b2b20218538