Add SkCanvasStack and update the Canvas utilities to use it.

BUG=
R=reed@google.com

Review URL: https://codereview.chromium.org/23865004

git-svn-id: http://skia.googlecode.com/svn/trunk/gyp@11081 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/utils.gyp b/utils.gyp
index df12b79..d3bac16 100644
--- a/utils.gyp
+++ b/utils.gyp
@@ -66,6 +66,8 @@
         '../src/utils/SkBitSet.h',
         '../src/utils/SkBoundaryPatch.cpp',
         '../src/utils/SkCamera.cpp',
+        '../src/utils/SkCanvasStack.h',
+        '../src/utils/SkCanvasStack.cpp',
         '../src/utils/SkCanvasStateUtils.cpp',
         '../src/utils/SkCubicInterval.cpp',
         '../src/utils/SkCullPoints.cpp',