Rename GrStencilBuffer to GrStencilAttachment

BUG=skia:

Review URL: https://codereview.chromium.org/1083133002
diff --git a/gyp/gpu.gypi b/gyp/gpu.gypi
index ada8c84..fe6631d 100644
--- a/gyp/gpu.gypi
+++ b/gyp/gpu.gypi
@@ -173,8 +173,8 @@
       '<(skia_src_path)/gpu/GrStencilAndCoverPathRenderer.h',
       '<(skia_src_path)/gpu/GrStencilAndCoverTextContext.cpp',
       '<(skia_src_path)/gpu/GrStencilAndCoverTextContext.h',
-      '<(skia_src_path)/gpu/GrStencilBuffer.cpp',
-      '<(skia_src_path)/gpu/GrStencilBuffer.h',
+      '<(skia_src_path)/gpu/GrStencilAttachment.cpp',
+      '<(skia_src_path)/gpu/GrStencilAttachment.h',
       '<(skia_src_path)/gpu/GrStrokeInfo.h',
       '<(skia_src_path)/gpu/GrTargetCommands.cpp',
       '<(skia_src_path)/gpu/GrTargetCommands.h',
@@ -295,8 +295,8 @@
       '<(skia_src_path)/gpu/gl/GrGLSL.cpp',
       '<(skia_src_path)/gpu/gl/GrGLSL.h',
       '<(skia_src_path)/gpu/gl/GrGLSL_impl.h',
-      '<(skia_src_path)/gpu/gl/GrGLStencilBuffer.cpp',
-      '<(skia_src_path)/gpu/gl/GrGLStencilBuffer.h',
+      '<(skia_src_path)/gpu/gl/GrGLStencilAttachment.cpp',
+      '<(skia_src_path)/gpu/gl/GrGLStencilAttachment.h',
       '<(skia_src_path)/gpu/gl/GrGLTexture.cpp',
       '<(skia_src_path)/gpu/gl/GrGLTexture.h',
       '<(skia_src_path)/gpu/gl/GrGLTextureRenderTarget.h',