Inline FloatRect::operator SkRect.

This is somewhat hot (about 0.8% of samples while painting glyphs), is
really trivial once inlined (a few moves and a few adds).

SkRect.h is a pretty lightweight header, so this shouldn't affect build
times very much.

Review URL: https://codereview.chromium.org/328163003

git-svn-id: svn://svn.chromium.org/blink/trunk@175974 bbb929c8-8fbe-4397-9dbb-9b2b20218538