gn win: disable warning C4706 to match gyp build

"Assignment within conditional" in dtoa and ICU code.

TBR=jamesr@chromium.org
BUG=354261

Review URL: https://codereview.chromium.org/417163003

git-svn-id: svn://svn.chromium.org/blink/trunk@179448 bbb929c8-8fbe-4397-9dbb-9b2b20218538