Add non-member non-mutating methods for common gfx::Rect operations.

cc/direct_renderer.cc

Index: cc/direct_renderer.cc
diff --git a/cc/direct_renderer.cc b/cc/direct_renderer.cc
index ac3ea3c3cb1abf10e51dae29a5f2cf4aca6cad98..d534c1ae0068953f2eb47b9719daf367b1743cb3 100644
--- a/cc/direct_renderer.cc
+++ b/cc/direct_renderer.cc
@@ -186,8 +186,7 @@ void DirectRenderer::drawRenderPass(DrawingFrame& frame, const RenderPass* rende
 
     const QuadList& quadList = renderPass->quadList();
     for (QuadList::constBackToFrontIterator it = quadList.backToFrontBegin(); it != quadList.backToFrontEnd(); ++it) {
-        gfx::RectF quadScissorRect = frame.scissorRectInRenderPassSpace;
-        quadScissorRect.Intersect((*it)->clippedRectInTarget());
+        gfx::RectF quadScissorRect = gfx::IntersectRects(frame.scissorRectInRenderPassSpace, (*it)->clippedRectInTarget());
         if (!quadScissorRect.IsEmpty()) {
             enableScissorTestRect(moveScissorToWindowSpace(frame, quadScissorRect));
             drawQuad(frame, *it);