Avoid unnecessary conversion when clipping

Core has many callers to to the IntRect flavor of
GraphicsContext::clip. Previously, the IntRect would be
converted first to a FloatRect, then to an SkRect. This
patch avoid the unnecessary FloatRect intermediary step.

Review URL: https://codereview.chromium.org/197313013

git-svn-id: svn://svn.chromium.org/blink/trunk@169267 bbb929c8-8fbe-4397-9dbb-9b2b20218538