Fix build breakage caused by r158725.

Remove out-of-date reference to FloatConversion.h

TBR=leviw@chromium.org
BUG=

Review URL: https://codereview.chromium.org/25741003

git-svn-id: svn://svn.chromium.org/blink/trunk@158729 bbb929c8-8fbe-4397-9dbb-9b2b20218538