Add `SkIRect bounds()` convenience method to SkImageInfo and SkBitmap.

Review URL: https://codereview.chromium.org/646213003
diff --git a/tests/ImageNewShaderTest.cpp b/tests/ImageNewShaderTest.cpp
index dfc5a24..1bc77fb 100644
--- a/tests/ImageNewShaderTest.cpp
+++ b/tests/ImageNewShaderTest.cpp
@@ -56,7 +56,7 @@
     destinationCanvas->clear(SK_ColorTRANSPARENT);
     destinationCanvas->drawPaint(paint);
 
-    SkIRect rect = SkIRect::MakeWH(info.width(), info.height());
+    SkIRect rect = info.bounds();
 
     SkBitmap bmOrig;
     sourceSurface->getCanvas()->readPixels(rect, &bmOrig);