add isClipEmpty() -- encourage clients to call this rather than checking clipFoo() results

BUG=skia:
R=robertphillips@google.com

Review URL: https://codereview.chromium.org/177073006

git-svn-id: http://skia.googlecode.com/svn/trunk@13568 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index 1e9dda3..8159adf 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -1484,6 +1484,10 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
+bool SkCanvas::isClipEmpty() const {
+    return fMCRec->fRasterClip->isEmpty();
+}
+
 bool SkCanvas::quickReject(const SkRect& rect) const {
 
     if (!rect.isFinite())