Fix compilation error of public/platform/WebFloatRect.h for MSVC 2013

This patch fixes compilation error of public/platform/WebFloatRect.h for MSVC 2013.

Error message on MSVC 2013:
d:\src\w\cr\src\third_party\webkit\public\platform\webfloatrect.h(111) : error C2039: 'max' : is not a member of 'std'

BUG=n/a
TEST=n/a; no behavior changes.

Review URL: https://codereview.chromium.org/28633002

git-svn-id: svn://svn.chromium.org/blink/trunk@159999 bbb929c8-8fbe-4397-9dbb-9b2b20218538